I just saw ridiculous speculation that trolls potentially evolved from dinosaurs which seems highly unlikely. IMO fen kreepers are a much better fit for an ancestral race. It is mentioned some types of trolls like the Amani can grow moss on their skin.
Most of the other races have some proto-species and explained origin but "prehistory" of trolls is curiously missing. There aren't that many options for their ancestry from what we know. It can be something connected to the Old Gods, something connected to elementals or something connected to Wild Gods/Loa. Of the beings I could think of bog beasts or something similar seem like the best fit, certainly better than something with tentacles or Rezan the dinosaur god.
I think possible connection is more obvious if you compare general build of bog kreepers and dire trolls. I know it's probably for technical reasons but in vanilla I vaguely remember they shared either same or very similar skeletons and even if they didn't similarities are more visible than in the build of normal trolls. We don't know exactly how dire trolls are made, but perhaps some kind of magic or junk magical chemicals can "awaken their ancient genetics" . In any case it's the best I could think of.
Been reading the Chronicle lately and became curious about trolls. Looked for theories of others but none of the offered explanations satisfied my curiousity. So I came up with this and thought to share it. It is what it is but I guess it can serve as my own headcanon for now. I'm not saying it's true, just throwing the possibility out there.