Hey Saffy! This is a quick one that can be answered, actually, because it's come up on Plurk in the past/we've discussed it.

More Monster choices will be added in the future, and there is a chance that preexisting Monsters can "evolve" into "matching" ones. ie; a Harpy into a Phoenix, a fox Turnskin into a Kitsune, etc. It's a mechanic that we know how to implement, but at the moment the game is locked to the feature, basically, until we get to a certain point.

But yes, Harpies do have to be based on real birds in the meantime!