OK, here are all the tree-monsters that i have written down (some are more treant-like than others):
Black Willow
Hangman Tree
Retch Plant
Twilight Bloom
Kampfult - see Tome of Horrors
Quickwood - see Tome of Horrors
Killer Tree - 6 HD, grapples with branches and swallows (OD&D/Mystara)
Noran - 9-HD, looks like a petrified treant, fires rocks from its mouth (Dragon Mountain/Monstrous Compendium Annual 1)
Tree, Singing - 12-HD lures creatures in with its singing to fall asleep, then drains blood (Monstrous Compendium Annual 1)
Bohun Tree - 10-HD NE "tree of death", shoot thorns, poisonous fruit, roots constrict, all-around vision (Dragon #89)
Clubthorn - 6-HD relative of holly tree has roots that entangle, and clubs with wooden limbs (Dragon #167)
Faerie Phiz - 10-HD creature, fey-like face appearing on enchanted wood, spit extinguishes flame and irritates skin, casts spells as both wizard and druid, has sagelike knowledge (Dragon #191)
also, in 3E books, there is the Ironmaw (FF), and Dark Tree (MoF) and probably others.
