Ents and Huorn are from Middle-earth, Treant name is from D&D since they couldn't use Ent name due to copyrights or some such. Treemen is just descriptive name to help find this tag.

Ents are generally more human-shaped while Huorns are nearly indistinguishable from regular trees, neither is stuck in some specific location like a regular tree, though they may choose to do so to disguise themselves as such and therefore avoid notice.