I recently saw this 1950s Hobbit short and, although its story hardly resembles anything by JRR Tolkien, the way it depicted the Trolls really piqued my interest. Instead of turning into stone in the sunlight, they turned into trees. This actually made a lot of sense to me, and I thought it was far more original than the one Tolkien came up with in "The Hobbit". Gnarled, old trees look a LOT like Trolls. A folk story about Trolls originating from or ending up as trees seems perfectly plausible and seems to actually fit really well with how they're depicted in D&D:
A troll's rubbery hide is moss green, mottled green and gray, or putrid gray...They are very wiry and lanky, but excessively strong and have menacing, dirty claws and sharp teeth. Trolls have the ability to regenerate damage they receive...A troll can reattach a severed member instantly by holding it to the stump....trolls cannot be permanently destroyed except by fire or acid. - Labyrinth Lord
I had never noticed it before, but that description sounds a lot like something made out of a tree. "Moss green", "wiry and lanky", "reattaching" to the "stump", destroyed by "fire or acid". WoW! I don't know about the rest of you, but I have never thought of Trolls as trees before. Most of the time, in modern media, they are depicted as oversized Orcs or Goblins.
This also got me thinking, though, that there could be different kinds of trolls - trolls that come from rocks (or hills), cave trolls, etc. Any natural outcropping of stone that looks like a troll could be thought of as their origin or their destination after death. But this idea could be taken to a lot of other places too. There's a place in eastern Utah called, "Goblin Valley", which I was very fond of when I visited as a kid, because the rock formations really look like goblins.
Both trolls and goblins and could originate from rock formations like this in and out of caves. I've also heard people use overgrown or decaying pumpkins as their source for goblin origins, and Orcs come from mutated or magically enhanced pigs and boars, fairies and pixies coming from flowers.