1E
As originally depicted, the umber hulk is an upright bipedal creature with a powerful, squat build. Allegedly, it's similar enough in form to a human to be mistaken for one from a distance in dim light; this seems a bit of a stretch even if you can't see the head, but perhaps the writers were thinking of something like ogres. The skin looks smooth, although it could have fine scales or sparse hair for all we know, and lacks such features as a navel or teats. The legs end in three-toed feet, with one toe a small claw to the rear that may help to grip onto rock, and the others broader and supporting the body weight. The hands have four clawed digits, notably including an opposable thumb, although there is no other indication of tool use.
The head is obviously the most distinctive feature. It's broad with a very short neck and has a wide mouth with reptilian-looking teeth and no apparent nostrils. More significantly, a pair of large serrated pincers, reminiscent of the mandibles of a stag beetle, project from the cheek region. These are stated to be used in attack, and could also grab and manipulate food as it is being eaten or bitten, but a pair of shorter curved tusks positioned medially to the pincers have a less obvious function. They don't even come close to meeting in the middle, so they can't be used in chewing or tearing at anything that could easily fit inside the mouth making it hard to see how they would be used. Finally, there are the eyes, with a close-set pair of vertebrate-like eyes in the middle and a pair of compound eyes positioned on the periphery.
Oddly, the umber hulk is not, in fact, umber, but is black over most of its body with a yellowish-grey patch on what's anatomically the venter. Since their heavy build makes it unlikely that they are agile, and they are certainly very slow-moving, the hide must be as tough as plate steel to give them the stated armour rating. They have their own language and are said to be as intelligent as humans. They live in what appear to be small family groups implying some degree of sociality.
2E
In 2E, the appearance of the umber hulk changes significantly. They are said to be 'scaly', but the picture only shows a thick hide with sparse hair (there may, of course, be scales too small to see). Heavy plates of what could be either bone or chitin surround the upper arms and thighs and the outer parts of the lower arms - those on the shins may, or may not, wrap around the back. There is an additional armoured nodule over the patella, and what look like scutes on the hands and fingers. Further plates cover the shoulders, the pectorals and the front of the abdomen - the latter including a depression that might contain a navel. All of this goes some way to explaining the armour rating, but it's also worth noting that the unarmoured parts seem to show vertebrate like musculature under the skin, and there's a suggestion of ribs on the chest.
While the hands have the same number of digits as before, including the thumb, the feet have six short claw-like toes that look like they'd have difficulty supporting the creature's weight - the posture shown and the length of the arms suggests that it might knuckle-walk like an ape to compensate for this. The head is entirely enclosed in armour, with the main pincers smoother than before, and the overall shape much narrower. Although we're told the eyes are all small, this only seems to be true of the central pair, which are, in fact, too small to make out whether or not they are vertebrate-like in form. The creature does, however, clearly have antennae, increasing its insectile look.
We're told that they give birth to litters of one to three young and that there is some degree of parental care by the mother. Although they retain humanoid levels of intelligence, there's still no indication of language or even tool use, let alone a meaningful culture.
3E
In a break with previous interpretations, the 3E umber hulk is now umber coloured. It is also considerably more insectile, with chitinous plates covering almost the whole of the body. Nonetheless, it still has ape-like hands and, of course, remains four-limbed. The limbs are oddly shaped, with massive lower portions and more spindly upper ones, something that would surely cause something of its size and weight difficulties in moving about. We're back to three toes on the feet, all of them short, clawed, and forward-pointing, giving the feet an appearance between those of the two previous editions.
There is more of a neck now, and the head has shifted to a globular shape with a much narrower mouth that makes it possible to use those inner mandibles to help it tear at and eat flesh. There are still no visible nostrils, although there is a row of what could be spiracles along the sides of the chest. The antennae are more prominent and the arrangement of the eyes is different, with the compound ones below and only slightly lateral to the smaller vertebrate-style ones.
Although it still moves slowly, it is slightly more agile than one might expect and its intelligence rating has increased slightly (although still close to the human average). It turns out to be about as strong as a troll, which makes sense for its stated size if not, perhaps, the lack of upper arm musculature.
5E
After the two previous changes in appearance, the umber hulk remains broadly similar in form in 5E. This version has more solidly built thighs and upper arms than that in 3E and there is more of a suggestion of ribs on the chest (these could, however, just be ridging on the chitinous plates). The head, does, however, shift towards a slightly more insectile look. Specifically, the umber hulk has labial palps beneath the jaw, although quite what they would be used for is unclear. Furthermore, the inner set of eyes, while retaining the position they had in 3E appear to be arthropod-style ocelli rather than vertebrate-like, with no evident pupils, eyelids, or canthi.
The statistics have, as usual in 5E, been toned down a little, but even so, the umber hulk is now stronger than a troll, and only just short of a hill giant. The intelligence has also dropped back to the original level. Umber hulks were specifically stated to be unable to speak in 4E, but they can in both 3E and 5E, regaining their own unique language in the latter. One oddity is that the tunnels they dig are eight feet tall but only five feet in width (2.4x1.5 metres), implying that they walk upright through them - one would have thought that they would just burrow them head-first, which would surely be quicker, but apparently not.
The umber hulk then, has become progressively more like an arthropod as its depiction has evolved through the editions. But, even if we ignore the earlier illustrations, this doesn't necessarily mean that it is one, since it obviously has some features that don't fit. The four-limbed form suggests a tetrapod, and the overall shape of the body is closer to that of a tetrapod vertebrate than is that of the similarly quadrupedal rust monster.
The forelimbs, for example, are divided into two segments, separated by an elbow joint. This contrasts with the four segments of an insect leg. The hindlegs are less clearcut, since they appear to have three segments, but its entirely possible that this is a digitigrade stance, with the metacarpals forming the third segment, as they do in dogs and other such animals. The arrangement of the digits on the extremities, especially the hands, support this since, again, they don't look at all insect-like and have same number of phalangeal joints as tetrapod digits. There is also no clear abdomen in the arthropod sense of the word, with the legs being attached at the hips, rather than further forward as they would be on an insect or spider.
If this is correct, it implies that most of the internal anatomy of the umber hulk follows the vertebrate form. There are lungs and a heart in the chest and liver, kidneys, bowels, and so on in the (tetrapod) abdomen. Furthermore, we would expect an internal skeleton that is merely supplemented by the external armour, much as it is in turtles or even armadillos. From a physics perspective, this makes sense, since a pure exoskeleton isn't likely to be strong enough to support the weight of something so large - of course, the existence of ankhegs and the like in the D&D universe implies that things might be different there, so it's not a guarantee.
It's with the head that this most seems to break down. Even so, it's notable that the jaws hinge horizontally, as they do in vertebrates, and each possesses triangular teeth, something they don't do in insects and the like. Countering this we have the "mandibles" which have no clear counterpart in any real-world vertebrate - they don't seem to be tusks like those of an elephant, for example. Thus, while it's plausible that there is a bony skull underneath the chitinous head-sheild, and one complete with a lower jaw, there must also be some major modifications.Perhaps there are additional bony plates in the angle of the jaw, jointed to the temporal and/or zygomatic bones, to which the mandibles are attached. Or maybe these are dermal plates of chitin, anchored by ligaments to the underlying skull, but not part of its bony structure. There must also be extra openings in the skull, to allow for the additional eyes and for the nerves and blood vessels that supply the antennae.
Attaching compound eyes to a vertebrate nervous system isn't a problem, anatomically speaking, since they still have an optic nerve that works in essentially the same way. Nor is it an issue that the umber hulk must have a double set of optic nerves, since these could attach to the brain close by one another. Similarly, the third, fourth, and sixth cranial nerves, responsible for eye movement, could also be duplicated, although they might not be if the compound eyes, in particular, are fixed in position.
Whether the antennae also need extra nerves depends on what they are for, since this differs in real-world arthropods. The umber hulk has no visible ears, so the antennae could be for sensing sound, in which case, given their position on the head in the 5E illustration, they may simply co-opt the usual auditory nerves. But then again, it has no nostrils, either, so they might smell the air, using, or replacing the usual olfactory nerves. It's also not impossible that the antennae both hear and smell, in which case, either set of cranial nerves is an option.
The lack of nostrils does, however, raise the obvious question of how the creature breathes, especially if we assume that the lungs are in the chest. Here, we must assume a significant departure from the tetrapod norm. There are basically two possibilities, one of which is that the umber hulk breathes only through its mouth, entirely lacking a nasopharynx and nasal cavities, but otherwise with a typical respiratory layout. The other is that the spiracles visible on the sides of the body in 3E and (less clearly) 5E are respiratory openings, implying that multiple short tubes run to the lateral surface of each of the lungs. This would have some advantages, since the animal couldn't choke, and would be rather hard to suffocate by physical action alone - there are a lot of openings to cover up.
If, on other hand, the spiracles aren't respiratory, they could still be sensory organs, since there's no reason that the organs of smell have to be attached to the breathing apparatus, as we see in a number of non-mammalian animals in the real world.
While the 2E illustration includes what appears to be a navel, implying that umber hulks are mammalian - a primate-like body with an insectile head - the later images do not. Thus, while they are tetrapods, umber hulks probably don't fit within any of the real world tetrapod classes; they're perhaps more like reptiles than anything else, but they aren't literally reptilian. For instance, living underground, in a stable thermal environment with insulating rock above them, they may well be cold-blooded and almost certainly won't have much need to regulate their internal body temperature.
It seems most likely that umber hulks lay eggs, or perhaps are ovoviviparous, holding the egg inside their body until it hatches but lacking a placenta or umbilical cord. None of the pictures show anything that's identifiable as external genitalia, but, even assuming the pictures aren't of females, they could well retract any intromittent organs into their bodies when not in use, so we can't assume that they aren't there, or in the usual place.
There isn't much to say about the umber hulk's signature power since it's so obviously pure magic. Early descriptions imply that it might have something to do with the odd-looking appearance of the creature confusing those who look at it, but it's not as if it's non-Euclidean, and, anyway, the effect is more potent than mere befuddlement (although less so 5E than it was in 1E). It's also notable that the creature needs to be conscious to use the power, which it is implied is under conscious control and can be turned off if necessary.
