Ok, this isn't really a theory, but more of some statments supporting each side of the argument.
Susan Strong is a Human
- At the end of Beautopia, Finn is suprised when he feels underneath Susan's hat. If she was Hyooman then he would probably of been disapointed.
- When Susan pulls Finns hat off, she is surprised that he has no gills. This could mean that she thought she was the only human and that she had no idea Finn was one too.
- Even though I don't think this counts, I'm throwing it in to get everything in here. Susan Strong has hair, all the others didn't. I don't really think this counts because it could be fake.
Susan Strong is a Hyooman
- Susan Strong wanted to swim to beautopia, suggesting she could have gills.
- When Susan pulls Finn's hat off, she was surprised that he didn't have gills. This could mean that she is Hyooman and thought Finn was too.
Ok, that's it. Tell me if you find anything else!