RTF is a global modification that replaces all towns in the game.

Yeah, there are unused IDs for the Yeti and Black Knight (this isn't the Death Knight). And of course, you can replace the Mummy, Manticore, and other neutral creatures.

Oh, that's great! I have a couple of ideas for campaigns. For one, we'll need neutral creatures (Ents, Griffins, maybe Panthers? and others), and for the other, Naga Destroyers.