Lords of Madness.
These monsters have been converted to 3.X, though sadly I can't say without needing (sometimes extensive) errata.
Leech, Magical: It's in Denizens of Darkness/Dread under "Leech, Witchbane."
Doppelganger Plant, Podling: It's in Denizens of Darkness/Dread under "Plant, Dread"...