After Yavanna heard that the Dwarves (Khazâd) of Aulë had been 'approved' by Ilúvatar, she felt afraid, for plants, her creation, were unable to defend themselves, she went to Manwë with her dilemma. She was given permission for her own 'forest protectors' by Ilúvatar -- walking, talking plants. The Ents regularly go back and forth between a state nearly like a tree and a full fledged 'ent'. There has always been enmity between Ent and Drarf, and Ents hate the sight of Dwarf axes. The assertion that Fangorn (Treebeard) is nearly as old as Tom Bombadil is strictly incorrect -- the Elves are older. Fangorn was "the oldest creature under the sun in Middle-Earth". Of course, we don't know if any of the original Avari were still alive.