1E
The original xorn has a barrel-shaped body covered with scales that basically take the form of roof slates, and are likely made of a similar, if less fragile, material. It is notable for its (mostly) triradial symmetry, something that makes it look particularly alien. It has three legs, which seem to lack knees and are similarly covered in the armoured stone scales - if anything, it's a wonder it doesn't move even more slowly than it does. The arms look to be less heavily armoured, and are clearly more flexible; they're positioned above the spaces between the legs rather than directly above the hips.
The toes are long and seem to be at least partially prehensile, although how much this would help given the stiff legs is debatable. Both hands and feet have three equal digits each, following the triradial theme. There are three large, slightly bulging eyes positioned around the body, one above each leg. They have horizontal pupils, like those of a goat, and readily apparent eyelids. The only feature that's obviously not triradial is the mouth, which is formed from two parallel jaws similar to those of a vertebrate... except that they're positioned at the apex of the body, which lacks anything you could describe as a head.
No other orifices or appendages are apparent.
It doesn't seem to be very strong, since its limbs do very little damage, but the bite is impressive for 1E, so the jaws are far more muscular than the arms. This may be to allow it to eat tough minerals, and it's notable that the teeth are flat and, to use the technical term, bunodont, which is exactly what you'd expect of something eating solid, crunchy, matter rather than biting into soft flesh. (We see something similar, for instance, in animals that feed largely on clams or lobsters). The armoured shell is particularly tough; superior to the hide of a red dragon or the coating of an iron golem.
While there's no hint of real society for the things, and they travel in small groups, they are of human-level intelligence and it's implied that they can communicate with at least some humanoids.
2E
The 2E version of the xorn is ovoid, rather than cylindrical, and has much longer, thinner arms than the one in 1E. The armoured scales are still present, but are much smaller and narrower than before, giving quite a different look. The legs are positioned closer to one another than previously, so that there isn't a flat undersurface beneath them. They appear to have ankles, but still no obvious knees, and the prehensile toes have been replaced with short, stubby claws that don't look as if they'd be of much use as either pitons or weapons.
The hands appear to have three fingers and a thumb, in a clear change from the previous look, but the biggest change is in the mouth. This is now triradial, with a high triangular 'jaw' above each eye, creating an overall Y-shape. The teeth are small and sharp and would probably be less effective at crushing up rock than the previous version.
In most other respects, the xorn has not changed in 2E, although we're told that they live in "clans". Whether this is intended to mean anything more than "small group" is debatable, since it may depend on how they are supposed to reproduce, which isn't stated.
3E
The look of the xorn has changed even more drastically in 3E. The body is squat and bulbous and gives the impression of being covered in a highly flexible, warty hide. It has lost its previous ability to camouflage itself against rock and has instead turned green. The legs are shorter than before, and while they do have both knees and ankles now, look rather uncoordinated - presumably they aren't, but the creature does look as if it hauls itself along the ground with some difficulty instead of walking in a stiff-legged gait.
There are only two fingers on each hand, and probably only two toes on each foot; the former have long claws, but the latter are back to their prehensile appearance. The eyes project out from the body on blobby projections, once again giving a very mobile appearance to the body rather than the solid rocky form it previously had. While the 2E version in particular looked like it might at least be passably effective at burrowing through rock (as it's supposed to) it's very hard to see this one doing so. The mouth is back to having bilateral jaws, but retains the small sharp teeth and a fleshy gullet with wide rubbery lips. Which seems a bit odd.
The armour has improved slightly as have the ability to use the claws as weapons - although they're still pretty feeble given the stated high strength. We're told that the xorn can sense its environment by detecting vibrations in the ground, which sounds a lot more useful on the Elemental Plane of Earth than its visual senses would be.
5E
The shape of the xorn's body in 5E is closer to that in 1E than to either of the other versions. However, the legs are no longer stiff, and the toes are stubby with strong claws. Rather than being positioned radially, as in the first two editions, the toes all point outwards, and there is a declaw facing inwards on the opposite side, as if it had evolved from some four-toed creature. The claws on the hands, which are more effective than in previous editions, are heavy, and the central finger is larger than the two to either side.
The eyes are proportionately smaller than before, and have vertical pupils, rather than horizontal ones. The hide of the creature is pebbly rather than scaled (or slated) and, while the camouflage ability has returned, the one in the picture looks more of a rich earthy brown than a rock-like colour, so perhaps it has emerged from soil or mud... or some kind of ore like limonite or bauxite. The mouth retains the traditional two-jawed shape, but the teeth are sharper and longer than before, looking even less useful for the purpose for which they're intended - great for biting into flesh, less so for grinding rock.
In many respects, the xorn in 5E has been toned down. Its base statistics are broadly similar, but the bite is noticeably weaker than it used to be and the creature has lost most of its ability to resist injury. It's also lost the automatic ability to speak Common that it had in 3E although this isn't really surprising given how rarely xorns are going to interact with humans.
Xorns are earth elementals and, as such, do not necessarily follow the rules of physics and biology that apply in our own universe. Having said that, their appearance, particularly in the form of their eyes, does suggest that they are at least partially biological as we would understand it, and may be composed in a similar fashion (in a way that, say, a regular earth elemental clearly isn't). But if so, their internal anatomy is certainly going to be odd.
With the possible exception of the 1E version, the limbs of the xorn appear to be constructed in such a way that they must have an endoskeleton, with muscles attached on the outside, as in vertebrates. Indeed, the structure of the limb is very similar to that of a vertebrate, especially in the later editions. Assuming that that's the case, there's probably a skeleton in the torso, too, but that obviously isn't going to be similar to our own. One possibility is a ribcage with three sterna, one above each leg/between each arm, connected by hoop-like ribs, some of which are modified to function as pectoral and pelvic girdles. The jaws at the top are presumably also bony (except possibly in 3E), but there may not necessarily be bones equivalent to a skull or spinal column.
Perhaps the most significant constraint on the layout of the internal anatomy is the fact that, in the absence of a head, the mouth is at the top of the body, with a centrally located digestive system below it. The anus, most likely, is in between the legs. A starfish is a good parallel here, since that has its mouth in the centre of its underside, from which food passes up into the stomach, and then out through a hole in the top. The xorn, of course, is the other way up, but the principle is much the same.
The xorn, however, has the additional problem that it must have a brain, and the lack of a head is a bit of an issue here. The most plausible solution to this is that the brain is doughnut-shaped, and wrapped around the throat. Most likely, it's somewhat triangular, with a lobe behind each eye and thick nerve cords connecting them, as well as others running down to the legs and across to the arms. There's no single spinal cord which is why I say it doesn't really need a spine (the nerve cords to the legs would be protected by the sterna anyway). This, again, has some resemblance to the neural layout in a starfish, although calling what it has a "brain" is probably stretching the definition a bit.
There is no evidence of nostrils on any picture of a xorn, but it's worth noting that they're always shown with their mouth open, so possibly they breathe through that - and, per 3E, even most outsiders do need to breathe. Under this theory, the nostrils are located somewhere in the throat; most likely three of them, each connecting to a separate lung rather than there being a single trachea.
Most other organs do not need to be present in triplicate although there's also no reason why they can't be, especially where, such as with kidneys, they are paired in most bilaterally symmetrical animals. The heart is an example of an organ that probably wouldn't be; you really don't need more than one. True, octopuses are often said to have three hearts and this is sort of true, but it's more accurate to say that the parts that make up the vertebrate heart are, in octopuses and squid, spread between three organs: a central one that performs the function of the ventricles, and two accessory ones that act like our atria.
If a xorn had an arrangement like this, it would actually have four "hearts" in total. This isn't impossible, but a single heart is equally likely, albeit, perhaps with six chambers in total rather than the four of mammals and birds.
On the other hand, it's also possible that it doesn't actually have blood, distributing energy through its body using metallic fibres or something of that sort. We certainly know that its body isn't composed of what we'd normally think of as flesh, even if it may have anatomical structures that are loosely like our own. The absence of blood would, for instance, explain why the 5E version is resistant to cutting and piercing weapons but not to blunt force trauma.
In earlier editions, xorns are also unaffected by extremes of temperature and can't be burned, and they're also resistant to electricity. The latter is probably them earthing themselves effectively, and the former to their alien composition which may be silicon rather than carbon-based. While this resistance is gone in 5E, it remains the case that xorn eat, not carbon-based lifeforms, but rock, much of which is comprised of silicates.
Crystals such as quartz may provide xorn with the basic nutrition needed to build their rocky bodies, but a possible use for the copper, silver, and gold that they crave is that these are what their nerves are made of, conducting electrical signals through the body in a manner that may, in some respects, outperform squishy organic neurons. Gemstones may provide them with rarer elements not commonly found in basic rock - beryllium from emeralds, chromium from some garnets, and so on. These may fulfil a function for xorn similar to that of vitamins in organic lifeforms.
How xorn reproduce is not stated in the core books. If they are sexual beings then telling the difference between males and females is likely tricky at best, but it could well be that they are not, reproducing parthenogenetically. Live birth and egg-laying are equally plausible from the little that we know although, since xorns do have some sort of culture, if only by sharing a common language, parents must raise their young rather than simply depositing them somewhere to do as best they can.
