There’s variations of it. two big ones I’ve seen. The main concept is the same: Trolls are pets for humans, and the stories genuinely deal with personage and rights of the trolls, or caretaking on the part of humans, or both.
Browse the petstuck tag and you’ll see two AUs with the same name- one is a bit more sexual in nature with more animalistic trolls- mainly those that line up with their zodiac creature. (For example, Tavros is a centaur-like creature with a bull’s body.)
The one that Loophole aligns with follows the UFUT model- that they’re not too different from humans, other than canonical features. Sometimes they’re drawn with features like European or Scandinavian trolls, with paw-like feet and tufted tails (I draw mine this way.) Sometimes they have hooves as well. (I’ve seen Equius and Aradia drawn like that.)