The 1E
Monster Manual includes several monsters that are essentially animate traps of one kind or another - floors that try to eat you, ceilings that try to eat you, stalactites that try to eat you, and so on. All of them with bland descriptive names rather than even an attempt at something atmospheric. The only one of these to remain consistently in the core rulebooks, however, is the mimic. Which is essentially a treasure chest that tries to eat you. It's certainly a contender for "silliest monster in D&D" (although it has some pretty stiff opposition) and it should come as no surprise that it's original to the game. Indeed, it's tailor-made for the "dungeon crawl" genre that D&D basically invented, since it makes even less sense in any other context than it does in that one.
So... yup, time for the mimic.
1E
As originally described, the mimic is a living creature that, for no particular reason, can disguise itself as anything made from wood or stone. Or, given that the picture clearly shows one pretending to be a treasure chest with rivets and a lock in it, presumably anything made from metal as well. It's possible that the fact that they're only encountered in pitch-dark dungeons occasionally illuminated by flickering torch-light means that they don't need to be terribly effective at imitating things to get away with it, but the implication is that they're pretty skilled - there's no indication given of a potential victim having any chance at all of realising they're fake.
They're probably less convincing to the touch, given that they have a hide equivalent to tough leather, which is hardly going to trick you into thinking it's solid stone. But, of course, they don't need to be, because, by the time you touch them, it's too late. We're told that they attack by extruding a pseudopod, which is exactly what we see in the picture, so there's evidently some flexibility to their body. The mimic in the illustration also lacks any visible eyes, mouth, or other evident bodily features because that would rather defeat the disguise, but presumably, it can form them as and when it needs them. After all, if it doesn't have eyes at all, it isn't going to be frightened of light, which (like some real-world subterranean creatures) it apparently is.
We're also told that there are two different kinds of creature referred to as mimics. One is comparatively large and has about the same intelligence as a dog. The smaller one is as intelligent as a regular human, and can speak its own language. Why it bothers when it's entirely solitary and will therefore never, under any circumstances, have anyone else to speak the language with, is a bit of a mystery, as is how it manages to learn all the other languages it can supposedly converse in.
2E
The illustration in 2E shows a more sophisticated form of pseudopod, ending in a pair of clawed tentacles, and has gone far enough into its attack mode to reveal a large mouth, lined with sharp carnivorous teeth and possessing a vertebrate-like tongue. On the other hand, it specifically lacks eyes, having photosensitive skin that can also sense body heat.
We're told that its hide is, in fact, stone-like and has a natural grey colour (this implies it must have a natural, resting, shape as well... but what it is we don't know). The armour class remains equivalent to tough leather, though, which may partly be because the thing can't really move about very much, but also seems to imply that the stony hide... isn't a very thick stony hide. Or perhaps it's brittle, like slate.
We're told that the non-sentient mimics are actively evil, which arguably raises some philosophical questions about the nature of evil. Oh, and mimics are immune to puddings. Which makes more sense in context than it sounds.
In 3E, the unintelligent mimics have disappeared, and the physical size stated for the creature matches that given for the smaller, brighter ones in the earlier edition. They weigh about two tons, which may sound a lot, but actually isn't when you consider that they have a volume of 150 cubic feet (4,250 litres). This works out to a density of just under 0.5 which is much less than we would expect for something made of flesh and blood... presumably, there are substantial air gaps inside it.
Their hide is tougher than before; it's hardly equivalent to plate steel, but it's on a par with mail armour, which is at least reasonably effective at warding off weapons. The text continues to describe their limbs as pseudopods, but the one in the picture is imitating humanoid arms, complete with fingers and thumbs and what appear to be claws. Indeed, the limbs look as if they have an internal skeleton... they probably don't, though, so they are likely much more flexible than they appear. Mimics are incredibly strong and physically resilient and their movement rate, while still dismal, is slightly better than it was before. They now speak Common, which at least allows some chance that somebody might be able to hold a conversation with one in its native language.
The mimic in the picture here is still pretending to be a treasure chest (we're assured they can disguise themselves as other things, but pictures of them doing so are comparatively rare). It still has pseudopods, although it isn't extruding them in the illustration, unless that's what the incredibly long tongue is supposed to be. It has far more teeth than in 2E, although, given that the form is mutable, that may not mean much. Oddly, though, it does have eyes - they must be concealed when they're closed, but that they're there at all is a change from 2E.
The hide has dropped down to being equivalent to thick leather again, and the speed has significantly increased, even if it still can't catch a running human. (Not having legs can't be a boon there). More importantly, the intelligence rating has dropped down to somewhere close to that of a gorilla - higher than the dog-like rating of the larger mimics in the first two editions, but well below that of the smart ones. There are apparently a few exceptions that are just about able to speak with humans, but they're rare, implying quite a downturn in the braininess stakes.
While it's just about possible to speculate as to what, say, a roper might be, a mimic, as described, is too weird to make any real stab at it. It's pretty clear that it doesn't resemble, even loosely, and real-world creature. Yet it is clearly meant to be a biological being of some kind, rather than a construct or demonic entity or whatever. In 5E, it isn't even an aberration, but merely a "monstrosity", a general term for living beings that aren't exceptionally alien.
In 2E, we're told that mimics have identifiable internal organs, so they aren't simply amorphous blobs that can take on other shapes. The fact that they're so mutable, and that their limbs are "pseudopods" counts strongly against the idea that they have an internal skeleton, and, in most editions, their outer hide isn't hard enough to be the short chitinous exoskeleton of an arthropod or calcareous shell of a mollusc, either. Not all molluscs have a shell, of course, and there are many other kinds of invertebrate in existence, some of which do, indeed, have a sort of leathery outer surface - although they don't have a habit of morphing into treasure chests or otherwise easily changing their shape.
The fact is, however, that we have nothing much to go on when considering what those internal organs might be, let alone how they might be arranged. There's a mouth, with teeth and a tongue, so there's going to be a digestive tube of some kind, although the mouth is so large that a stomach seems unnecessary. By the same logic, in addition to the orifices we know of, there must surely be an anus concealed somewhere on the body. This also implies excretory organs of some kind, although they don't necessarily need a separate opening.
The thing must breathe, and its size suggests lungs and a circulatory system with some kind of pump - although neither would have to look like the vertebrate versions. The nostrils appear to be concealed, so it's likely that the animal can hold its breath for extensive periods of time while it's inactive - something that's quite plausible when you consider that whales can do so for hours on end while swimming and actively searching for food, rather than just sitting motionless waiting for it come past.
The mimic's intelligence may vary, but it's high enough for it to require a brain and a nervous system, whether or not it also has eyes. Other organs are certainly possible, but the arrangement and/or shape may be alien enough to make some of them hard to identify.
Much of the internal body is likely composed of muscle, in addition to any air spaces to lighten its body as the 3E weight implies. But one of the things that's most notable about the mimic is its ability to change its shape, so whatever organs it possesses must be able to be compressed and re-arranged to suit its needs. There must be some limitations on a mimic's ability to, well, mimic, but we know that it can't change its overall volume or body mass, and this is significant when we consider that, supposedly, its favourite form other than a treasure chest is as a door.
Since the door doesn't need to actually open, it's probably much thicker than an actual door, extruding itself into whichever space it assumes potential prey won't be advancing from. But even so, it's surely flatter than a trunk, meaning that it can squeeze its organs into a new shape and arrangement when it needs to. Quite how far it can do isn't stated in the core rules, although something like a very long and narrow rope is probably beyond it without giving itself the equivalent of a hernia.
Mimics are said to be completely solitary - even in 1E, which often has creatures living in small groups. So they're probably asexual. 2E confirms this, and says that they reproduce by fission, which is a bit unlikely for anything so large and internally complex. However, some kind of asexual budding is a possibility, with the offspring being concealed inside the body until it is large enough to be extruded and wander off on its own.
The ecology of the mimic is, of course, completely bonkers given its extreme artificial specificity. 2E states that mimics were originally created by wizards and so were specifically designed as organic traps to place in the subterranean complexes that such people always feel the need to construct. What the heck they created them from is a bit of a mystery, but that they aren't constructs even to the extent that a flesh golem is implies that it must have been something (or several somethings merged).
If we're willing to accept that wizards go around building dungeons and that they assume that treasure-seekers will break into them at some point and be distracted by a fake chest then... sure, making one that eats people isn't totally unreasonable. You can imagine that the Ancient Egyptians who put traps in their tombs to stop robbers might have done something like this if they thought they could get away with it... and if they could supply enough food for the mimic to survive on during the long waits between break-ins. (5E says that, looking as they do, mimics can be assured of "a steady stream of prey". Dude, if your home is getting broken into that frequently, you should move to a lower-crime neighbourhood. We know you can afford it).
But how do mimics get from one dungeon to another? They don't seem to have the ability to pretend to be anything other than furniture, which is decidedly limiting as a form of camouflage. Yet if the things live worldwide, and aren't being created all the time, as golems and undead are (and even 5E, which is normally quite keen on this sort of explanation, doesn't go there for mimics) then travel they must. And know where to go.
Perhaps some dungeons-are-us delivery service is shipping them across the world to their clients.
