An Alternate Ecology of Displacer Beasts and their Kin.

The displacer beast is a classic monster in D&D lore. Portrayed as a large black feline, like a panther, but six legged, it has two long tentacles growing from behind its shoulder blades. These tentacles are tipped with spiked pads covered in teeth. But the most significant thing about the displacer beast is that it somehow appears to be several feet to one side of where it really is, thus 'displacer'. The official explanation of their displacement seems to be subconscious creation of a magical illusion that bends the light around them, an illusion to which they themselves were immune.

There are other potential explanations, of course. The illusion could be psychic in nature, simply convincing onlookers that their position is different than it is in reality. They could be slightly out of phase with reality, their actual position shifting somewhat randomly within a small space, like a full size example of Heisenberg's uncertainty principle: they aren't anywhere specific until interacted with physically, when their actual position is discovered by direct observation. That last possibility leads us to the main thrust of this article, which is that displacer beasts are neither beasts, nor monstrosities...but rather only a single example of what happens when a specific type of invasive aberrant symbiote or parasite from the Far Realm infects material creatures. There are plenty of examples in the real world of such creatures becoming fully symbiotic in ways that allow them to propagate in that form without being infectious or needing to infect . . . much of human intestinal flora and fauna fall into that realm.

We know, even in canon, that displacer beasts, specifically the common feline form, are not the only kind. Displacer serpents also exist. I posit, of course, that many varieties of displacer beast exist. Cats, hounds, serpents, elk, and more. Like many scions of the Far Realm, they seem to have a deep and instinctive dislike of more natural creatures, and of fey, explaining their antipathy toward blink dogs.

Another interesting question is, how does a normal brain handle coordinating all those limbs? 6 clawed limbs and two tentacles that operate entirely differently, all managed by a mammalian brain? Well, the answer is . . . it doesn't. Like an octopus, those tentacles have their own 'sub-brains' nestled in the lumps at the base of the tentacles, and may be the most purely alien, aberrant, part of the combined creature. It is those brains that handle the displacement effect (however you decide it works, whether magically, psionically, or quantum uncertainty) as well as controlling the tentacles, allowing such creatures to fight with claw, fang, and tentacle all at once . . . but occasionally resulting in independent, and odd, actions and reactions by the tentacles.

As such, one starts to think of 'displacer beast' as a template that can be added to any Prime Material creature, allowing any animal, or even humanoid, to become such a thing, merely adding (for 5e) the Avoidance and Displacement abilities, and the Tentacle attacks at an appropriate damage level for their size.

I do have examples on the website of other displacer creatures who might have more significant changes. Links below.

https://www.ofgodsandgamemasters.com/dnd5eandblackflag/dnd-donarsday-the-greater-displacer-beast

https://www.ofgodsandgamemasters.com/dnd5eandblackflag/extra-dnd-monster-displacer-hound

https://www.ofgodsandgamemasters.com/dnd5eandblackflag/extra-dnd-monster-displacer-elk

https://www.ofgodsandgamemasters.com/dnd5eandblackflag/displacer-dragon-fan-content-5eblack-flag