The animal theory seems to hold the most weight, of course. One has to ask though what gives them their green pigmentation. We might further ask if Kerbals are even carbon based life-forms.
Biology is something that others will find more interest in with the kerbals, I personally am interested in them from an anthropological perspective. What is their history, their culture? It seems that all that we know is that they are advanced enough in technology to support a robust space program, and that they have been interested in spaceflight and astronomy for some time, as is our species, but they seem to perhaps be even more engaged in it.