Tree-Halla Guy reminds me of Varterrals. They're an elven construct of people, trees, and whatever else ancient elves used to make their own golems with. It's not very clear with the one you fight in DAO, but the concept art shows more defined body features like legs, head, etc. They're supposed to be ancient guardians of elves and their temples.
If elves could make those, why couldn't they also have Tree-Halla Guy and Demon Flower Girl as other creatures they created? Maybe Ghilan'nain has something to do with those, or maybe June with his craftiness. Zathrian bound a wolf and a forest spirit together. Those guys who were in Amgarrak tried to bind a spirit from the Fade into the body of a flesh golem.
Ancient Elves were into some weird ****. Ain't got nothin' on Tevinter.
<edit> I think this is the concept I'm thinking of. Could have sworn there was one for DAO as well because Witch Hunt. The achievement for killing the thing is even titles "That Thing Has Legs."
