|Actually a photoshopped jaguar...|
First EditionAs shown here (and as described in other early editions), a displacer beast resembles a puma, but with two pairs of hind limbs and a pair of large tentacles sprouting from above its shoulder blades. About half the length of the underside of each tentacle is covered in sharp horny growths, allowing it to use them as weapons. The animal is blue-black in colour, with luminous green eyes, and, like a lion, but unlike a puma, has a tuft of fur on the tip of the tail. Physically, it is larger than a puma, being about the size of a lion or tiger, and has a tougher hide. It hunts in small packs or prides, and has about the same intelligence as other big cats (which is actually slightly higher than that given for most wild animals). Given this apparent lack of sentience, it unsurprisingly has no alignment or moral compass, although it is said to "hate" all other life, presumably implying that it's just plain vicious.
Second EditionThe animal looks much the same as before, although this time the tail lacks the tuft at the tip, making it look more puma-like. Like a cat, it is explicitly a furred animal. We are given some more detailed information on their body length and weight, which confirms that they are about the size of a tiger, with males being larger than females. This time, they are said to be evil, which seems to mean little more than that they sometimes kill for the sake of it, even when they aren't hungry. There is also considerably more information on their reproduction and pack structure.
Third EditionThe change of appearance in the displacer beast is significant, with it now becoming a remarkably emaciated animal with stick-like limbs. The text states that it still has blue-black fur, although the illustration looks as if it is entirely hairless. A row of small spines runs down the backbone, and there are further spines on the elbows, knees, and ankles. The horny growths on the tentacles are now restricted to a leaf-like extension at the tips, and the eyes are red, instead of green. From an anatomical perspective, however, the most significant change is that it now has two pairs of forelimbs, rather than doubled hind-limbs. In other changes, the animal can now "see" in the absence of light, and has sufficient intelligence to be able to speak human languages; the latter also suggests that the mouth and voice-box are at least somewhat human-like.
Fifth EditionWith this edition, we return to something much more akin to the earlier appearance. The arrangement of the limbs and the shape of the tentacles remains as in 3E, but the creature is now sleek but muscular, once again resembling a puma in form, rather than a starved greyhound. The fur is extremely short, but does seem to be present. While it retains a slightly higher intelligence than in the first two editions, it no longer has the ability to speak.
Setting aside the slightly odd appearance given in 3E, we have an animal that resembles a six-legged puma/cougar/mountain lion with tentacles sprouting from above its shoulders. The primary point of disagreement seems to be whether it is the hind limbs or the fore limbs that are doubled up. I'd argue that, anatomically, the latter is the more likely. This is because a doubled pectoral girdle (that is, the shoulder blades and clavicles, presumably articulated with an elongated sternum) poses no obstacle to the positioning of the lungs and heart. It is much harder to see how the internal organs would be arranged in an animal with two sequential pelvises.
The flexibility of the tentacles implies that they do not have an internal bone structure, and are pure muscle, like those of an octopus. Most likely, they attach somewhere between the two pairs of shoulder blades, in order to allow for the necessary nerves and blood vessels to pass between the bones. The glowing eyes would, in reality, be impractical, since they would surely obscure the animal's own vision, rather like ears that emitted a constant buzzing noise.
In other respects, we can assume that "unusually large puma" is a good template for the animal's general anatomy, and, where there's no reason to assume otherwise, its biology and behaviour. Sometimes this may be a little arbitrary; for instance, it implies that, like pumas, but unlike tigers, displacer beasts cannot roar. Since this is related to the detailed structure of the throat, which isn't in evidence, it isn't necessarily so, but there's nothing to contradict it, either, so it's a fair starting assumption.
On the other hand, the feline head strongly supports the claim that this is a purely carnivorous animal, since the skull of cats is shaped the way it is for precisely that reason, allowing for powerful jaw muscles and a relatively small number of meat-slicing teeth. Like cats, the displacer beast is likely to be an ambush predator, leaping from cover, rather than pursuing its prey for long distances. This is the way most cats hunt, and only the 3E version looks even remotely like a cheetah. We can also note, of course, that the stated speed for the animal is similar to that of other big cats, and slower than, for example, a wolf.
The habitat of displacer beasts is generally stated to be either hills or mountains, and broadly in the temperate zone. It seems likely that this refers to the ruggedness of the terrain, rather than specifically to altitude, which, again, would fit with a puma more than, say, a snow leopard. 5E implies a more wooded habitat, although, of course, that isn't inconsistent with living on steep slopes and rugged hills. Given their large size, their diet may be similar to that of tigers, with deer and wild boar predominating when they can't snack on passing adventurers, although perhaps with a higher proportion of wild sheep, and even goats, in the hills.
It's also clear in all editions that the primary weapons of the displacer beast are its tentacles. This makes sense, since they'd have a longer reach than anything else at its disposal. Realistically, they would be used to grapple onto prey, digging into and raking their flesh with the horny tips as the target struggles to escape. Once the victim has been forced down, however, the beast would probably attack as cats do, tearing open the abdomen with their four sets of fore-claws, and suffocating with a bite around the neck. The tentacles would also help it to drag the carcass away to eat in seclusion.
The intelligence of displacer beasts increases steadily across editions, until by 5E, they are more intelligent than ogres. Mind you, so is a gorilla, so this is probably still intended to indicate no more than a high degree of animal cunning. Once again setting 3E aside for these purposes, displacer beasts might have quite sophisticated means of communication - cats can produce a range of different sounds - but nothing that's complex enough to constitute true language, just enough to convey basic intentions or the location of hazards/prey.
Almost all cats are solitary animals. Pairs of siblings might occasionally stay together for a short time before reaching full adulthood, but otherwise, cats normally remain alone once a mother has finished rearing her young. The one exception, of course, is lions, and it's notable that displacer beasts too, appear to be social animals. In 3E, displacer beast packs are similar in size to lion prides, and 5E gives no indication of numbers, but the earlier editions make them slightly smaller. Even so, there are multiple adults, and, if lion prides are any indication, there will typically be one or two breeding males, or even three, in the larger packs, with the remainder made up of adult females and yearlings.
Dominance based on strength and ferocity seems likely, from what we know of them, which, if the males are larger, makes them the pack leaders. Although the males within a pack must cooperate to some extent, they are likely territorial when it comes to outsiders of their own kind. While the size of the territory would vary enormously with both pack size and the richness of the local ecosystem, based on lion prides, between 40 and 160 square miles (100 to 400 km²) is perhaps a rough guideline for what would be likely.
1E states, rather strangely, that only adult displacer beasts are ever encountered. With 2E, however, we have some indication of their reproduction, with litters of up to four cubs being born annually, and initially raised in a secluded lair away from the pack (a cave is stated as being typical, and tigers will indeed use caves for this purpose if they can find any, but any shelter would probably do). The given litter size is quite plausible for big cats, and the length of pregnancy is probably similar to that of lions or tigers, at around 110 days.
2E also states that the cubs are born without their tentacles, which seems about as likely as elephants being born without trunks. Rather more believable would be that they initially lack the horny spines, which perhaps begin to poke through at around the same time as they start teething, since they won't really need them until they're starting to be weaned. Newborn displacer beasts are probably also blind and helpless, much like kittens, but that won't last long. They are said to become independent hunters at six months; it takes eighteen months for a tiger to fully reach this stage, so that's perhaps an underestimate.
Which leaves us with the "displacement" ability itself. The game mechanical effects of this vary between editions, but the description of it does not: a displacer beast visibly appears to be in a different location than it actually is. This is initially defined as being three feet (one metre) from its true location, in a random direction, but the distance is vaguer, and probably shorter, in later editions. Quite how this works with regard to things like the animal's footprints is unclear, but it does appear to be a genuine warping of light, rather than some mental effect. 2E states that it takes the cubs four months to manifest the ability, so it's not something inherent in their physical composition.
When you think about it, this is a rather odd ability for a predator to have. It would be very useful for an animal like a deer, which could use it to avoid attack, but, unless dragons or the like are really common, displacer beasts are likely to be the top predators where they live, and unlikely to be attacked by wild animals. It might help a little in terms of their ambush tactics, since prey animals will be looking in the wrong place if they are trying to keep an eye on one, but even then, the distance involved isn't likely to make much difference - once the beast is visible, fleeing is your best option regardless, and it should at least be obvious which way is "away".
It's not much use in defence against cornered prey animals, either, since by the time the target is grappled, it's pretty obvious where the beast is. (Indeed, some editions specifically state that the power doesn't work on a grappled creature). Of course, it is useful against humans, especially those armed with missile weapons or spells that need careful aiming, which is the whole point of its existence in D&D. I'd argue that the most likely explanation, therefore, is that displacer beasts were deliberately given their power to serve as guard animals of some kind, perhaps by godlike beings (such as the Unseelie Court) who wanted to give them to their servitors.
[Photo by Eduardo Estrada.]