Scarlet's Game Room
I know of 4 creatures that drop Treant. There could be more, I don't have the scoop on ALL the bosses out there yet. I been trying to get Mandy to do my work for me - uh, I mean HELP me, but I haven't had much luck yet.
Telegraph Box (Zafaria), Zafaria
Hamakala's Mausoleum, Krokotopia
Village of Sorrow, MooShu
Pagoda of Harmony, Krokotopia