The pygmy cave troll is not actually a type of troll — or not the way we typically think of such things, anyway. They are called this because they resemble trolls, and regenerate like trolls, but are usually the same size as kobolds and troglodytes.
They are actually a type of magical fungus, created by long-dead archmages driven mad by the arcane energies of other planes. This fungus has the ability to absorb and replicate the genetic material of other living things it consumes. This is not a particularly fast process, but over the last thousand years or so it has resulted in the fungus achieving its present form. Pygmy cave trolls reproduce by budding, and must continue to consume living things in order to profit from their abilities. They have absorbed so much more of the essence of trolls precisely because of the magical regenerative qualities of those creatures — there is always more for the fungus to consume. If pygmy cave trolls are not able to consume trolls or similarly evolved creatures, their mimicry of higher life forms degenerates. Even eating other pygmy cave trolls is not enough.
Aside from regeneration, which is partly due to the innate ability of the fungus to reform itself, pygmy cave trolls have achieved a sort of bioluminescence and a rudimentary animal intelligence.
This same bioluminescence is a characteristic of many of the underground lairs where the pygmy cave trolls dwell. It is produced my molds and fungi that grow upon damp stone walls. Not only does this light interfere with nightvision, but it changes the perception of colours in near-random ways. Any colour-based decisions made under the influence of this bioluminescence will be little more than guessing games — violets appear blue, black clothes appear to glow yellow, a sprightly green jacket seems washed in ugly purples and artificial orange. These colour-warping properties can shift over time — even as the light given off seems to stay the same — due to the frequencies of non-visible light that these growths give off in conjunction with their bioluminescence. Unexpected interactions with magical effects can also occur.