Right now I would suggest that tagging can work for that purpose. Just tag them as both "feral" and "humanoid," and also tag the other characters accordingly. That way you can click on either one of those tags to see all the characters with those forms.
I thought it would be really helpful if we could categorize characters in our TH into more than one folder
For example, I have a character that has both a feral and humanoid form so I would like to have them appear in both my feral and humanoid folders
Something like that, anyone have any thoughts on this?