I believe that the existing, non-aliased tags should be implicated to monster_girl, and those which currently allow costumes should have those images be re-tagged to change them to animal_costume and other matching tags. For example, change "bunny_girl" to "bunny_suit" in existing images where they're not actually monster girls, and keep the others.
Lamia and other monsters should also be implicated to monster_girl as well. Does anybody else believe that this is something worth doing?