Fantasy and Urban Fantasy biology is one of those things that just doesn't stand up to much scrutiny. Nature science does all kinds of fun and fascinating things all on its own and in the real world, and a lot of that can be passed on or extruded into fiction, but when it comes to choosing between the demands of the plot or the demands of fictional biology, the plot wins 9 times out of 10. And this is why, in books or on TV, nobody poops. Unless it's funny.
Wesen biology is something we've all been fascinated by, especially with the revelation that there can be not only naturally born and viable Wesen and Wesen-human hybrids, but also that the genetics can be manipulated and mutilated to create an artificially engineered chimera-style hybrid. And how does the morphing ability even work, physically speaking? How the hell does the digestive system of a wendigo manage nails and tough skin, or was that just extra flavor to be carved off later in the dining process? How do Reinigen control their rats? What the hell is the Seltenvogel throat-bubble thing all about? The questions go on and on. Twenty years ago, your dear blogger kept a journal of counterpoised X-Files cases with real life cases from which the X-Files writers might have taken their ideas and details. Now, we'll do something similar, and reverse engineer some theoretical Wesen biology out of what we're given to work with from the show. When all else fails we'll resort back to magic, because obviously that exists too, and must be considered.
For structure, we'll take it in alphabetical order, lacking the usual correlation convention of herbivores = good and carnivores = bad. Observations based on existing show data will go first, followed by pure speculation.
Bauerschwein are one of the many species we don't get much data on as far as non-human abilities or traits go. We see one on-screen example and hear of a couple of others, but the one on-screen example does little other than woge (which for the purposes of this essay we're treating as a universal either magical ability or as a visual convention signifying that this person is a Wesen) and have a mud bath in which he is entirely submerged. It doesn't take an other-than-human ability to hold your breath while submerged, especially if you're the one controlling the depth and duration. It also doesn't take an other-than-human affliction to cause a desire for a mud-bath; many thousands of people indulge in these for health, beauty, or relaxation purposes. Canon information now dispensed, pigs are (as anyone who watches far too many forensic crime dramas knows) very similar to humans in a lot of respects. The most distinctive aspects I can think of would be the dense musculature, perhaps leading to a greater-than-average strength as compared to a human, and a sturdy digestive system. By contrast, pigs have small lungs relative to their bodies and are susceptible to fatalities resulting upper respiratory diseases, so pneumonia or bronchitis might be more of a plot point in a Bauerschwein-focused story.
We have several examples of Blutbaden to choose from, the main ones being Monroe and Angelina. In the first episode we are (presumably) reliably informed that they engage in pack behavior, that they possess better than human sense of smell, that wolfsbane masks the scent, and that they are incited to violence by the color red. The first is given nothing specific to categorize it; many, many species engage in group behavior up to and including humans, and what that practically means is probably just that groups of Blutbaden are more subject to mob mentality than most. Over time, any group of humans will eventually settle around a leader and one to three secondary leaders if the alpha is absent, so nothing very new there. Possessing a better than human sense of smell seems to, sadly, come and go with the plot, but it's a standard thing for a werewolf type creature to have, Blutbad or otherwise. Biologically, this happens because a canine/lupine nose has around 25x the olfactory receptors of a human nose. Fictionally, it's possible that Blutbad olfactory receptors are both more numerous and more efficient than human. We also have an example in the first episode of Monroe marking his territory with urine as a canine would do, but that seems to be more for comedy value and a voluntary action/precaution taken by the character than a strong biological impulse. We don't have any information on whether or not it's common among Blutbaden, or even Blutbad males. There is absolutely no correlation apart from the name between wolfsbane and masking or removing any scents at all, let alone those of a wolf or wolf-related creature. As for the color red, I've got nothing. There is some science that indicates that surrounding colors can be factored into overall moods, but nothing so simplistic as "see a red shirt, feel belligerent." So. For the most part, this covers the usual biological quirks and additives associated with werewolves in speculative fiction all over the place. Since our recurring Blutbad is a male vegetarian we don't get into the subject of multiple births (likely, given the number of relatives) or the necessity of eating meat on the rare side. The only other aspect that might eventually be covered within the show would be body language, which is important across all animal groups and yet distinct for each species or genus and, in the case of lupine body language, relatively well-known.
Damonfeuers are one of the more disgusting bits of biological innovation I've seen lately. The mechanism for fire breathing is essentially that they vomit up liquified fat and then somehow ignite it, causing not quite Michael Bay level explosions, but close enough. There are a number of problems with this theory once you stop taking it as gross science fiction biobabble and looking at it closely. Beginning with the liquified fat, this would most likely cause Damonfeuers to either run dry quickly and not have much explosive capability overall or they would need to process carbs, sugars, and food fats into body fat which they could then liquify very, very quickly, giving them a prodigious appetite. Both is most likely, really; any kind of process they would have to render food into liquified fire-ready fat quickly enough to be able to replenish their stores while in combat would also heat their bodies to the point where they themselves, most likely, would be in danger of exploding. From there we can move on to wondering what the hell the ignition device is. The ignition temperature of human fat is several hundred degrees Farenheit; you would have to heat the liquified fat to at least a couple hundred degrees to get it to ignite from a heat-based ignition source, significantly beyond human norm and therefore noticeably hot to the touch and unable to mingle within non-Wesen or even most Wesen society. The ignition source must therefore be chemical, as with the bombadier beetle, a mixture of substances generated within the body that are combined only at the instant of ignition. Where they're concealing those glands, though, apart from on their face, is a mystery.
Drang-Zorn are another relatively easy one; apart from the woge their chief trait in their depictions so far is the puberty from hell. There are a number of descriptors of the chemical processes within the body in general and the brain in specific that describe biological causes such as brief surges of strength, poor impulse control, and, again, puberty from hell. WIth only one family of examples to go on, it's hard to say how much of what we see in that family is specific to the individuals and the situation or is common to most Drang-Zorn. One thing that does come to mind is that if, as is strongly hinted at within the episode, the impulse control issues are exacerbated at puberty, if most Drang-Zorn females go through at least the emotional lability commonly associated with PMDD at every cycle.
Eisbiebers also have no significant extra-human abilities that we know of, although they seem inclined towards construction or maintenance as a field of work. This may be a result of greater-than-human-average hand-eye coordination and dexterity, and a brain geared towards spatial reasoning. By the same token Fuchsbau might be geared towards recognizing microexpressions and interpreting them with a natural-born talent, or interpreting body scent and pheromone output via an increased olfactory sense like the Blutbaden.
With only one family as an example (and one of those family members severely compromised in the biology way) it's hard to say what if any extra-human talents the Genii Innocuo have. The only species trait mentioned is a strong tendency toward pacifism, which is fairly common in both herbivores and heavily isolated groups. Since they supposedly originate in the Galapagos Island, it's not unreasonable to assume that they do have some sort of evolutionary oddity, as most of the residents of the Galapagos Islands do. The Geier seem to have evolved only one small physical trait, that being the retractable claws. Assuming these function similar to feline claws (which is somewhat out of line with their vulture traits, but we'll bypass that), it wouldn't be difficult to restructure Geier hands to be of a similar size to human hands and still contain retractable claws. It would still create hands which don't resemble human hands in certain key ways, but that could be gotten around with gloves (medical gloves) or other professions where the hands are disguised, kept out of sight, or kept moving too much to be examined closely.
Initially both of the Wesen we're shown, in the first fifteen minutes of the pilot no less, are fairy tale creatures. The Hasslich representing trolls and the Hexenbiest representing witches, neither of them contain extra-human abilities that can be explained by science. In point of fact, the Hasslich, other than being hard to kill (or so we're told) don't appear to have any extra-human abilities at all. We can put down being hard to kill to increased reflexes, muscle density, etc., all things that are easily concealed in a human-appearing shape. The Hexenbiest's abilities, by contrast, appear to be entirely magic. Which we are not touching. There might be some blogger out there made of sterner stuff than I who is capable of analyzing the details of how each potion affects each individual to achieve the desired result given in the show, but that is not me. Plus the effect that happened when Adalind's witch-half died. I have no scientific explanation for that and I'm not even going to try.
Hundjager, though, like Blutbaden, are easy to describe. Better-than-human musculature, possibly denser bones as well depending on how much punishment they're expected to take, increased olfactory sensors, and an adjusted number of rods and cones and optical nerve fibers to give senses more attuned to hunting than humans are, and you probably have a Hundjager. Given that we have only a couple examples and only one developed one, it's difficult to say whether or not their inability to tactic their way out of a wet paper sack is a result of lack of a strong alpha or just poor quality of mook in general. The former would indicate a brain structure not unlike that of many kinds of insects, one that increases in productivity when there are more of them. The latter would indicate Adalind's shit ability to judge a good mook, or possibly that Eric doesn't give a Reinigen's ass if she succeeds or fails. We can throw in Jagerbar with the Hundjager on the lines of a more efficient physical construction, possibly including greater lung capacity and an inclination towards barrel chests, or more efficient capacity to oxygenate the blood. The scene with the running in the woods certainly implied increased stamina, but that might also have been a product of scripting and blocking rather than a conscious decision to make the bear Wesen better/stronger runners than humans.
Klaustreich aren't given any extra-human abilities either; if I were making a Klaustreich, other than their various personality disorders (some of which might be tied to similar hormone imbalances to the Drang-Zorn, albeit in decreased amount) I would restructure their eyeballs to include the extreme pupil dilation/contraction typical of cats and adding a tapetum lucidum, which is the reflective coating just behind the retina that enables cats to see even in very little light. It diminishes their daylight acuity, so perhaps a decreased reflectivity in the tapetum would balance that out. Offhand I can't think of too many other hunting advantages a cat has that could be easily concealed in a human looking body, so we'll leave it at that. The Konigschlange and its cousin the Lausenschlange also have more in the way of personality traits than physical traits; their cold-bloodedness is more ruthlessness than physical inability to regulate temperature. It would be possible for a human-looking mouth to conceal poison sacs perhaps where wisdom teeth would otherwise reside, it would perhaps be possible to be cold-blooded and it might well be possible to connect the sensory receptors in the tongue to olfactory receptors for the additional sensory input power, but we've seen no evidence of any of this.
By now you've probably noticed the trend that most of the Wesen follow, namely that there is little in the way of physical differences and most of the species markers are personality traits rather than biological differences. Honestly, there's probably an essay at least twice if not five times this long on how that reflects on nature versus nurture, what's learned and what's enforced by stereotypes and societal norms and what's biological imperative. The middle ground essay would deal with whether or not the different perceptions each species has of the world via their biological differences colors their ultimate behavior and outlook, such as the Drang-Zorn being impulsive and angry because of hormonal imbalances, or the Klaustreich being perceived as manipulative because they're more attuned to miniscule movements, micro-expressions and gestures, than other creatures and thus respond more readily to non-verbal cues. That doesn't explain the bastardry, of course, but generally the louder and meaner examples get more reputation more quickly than the gentler, benign examples. At any rate, that's a sampling and a beginning of looking at the potential underlying biology of Wesen. In a couple of days we'll have a look at the rest, inasmuch as the show gives us anything to look at.