There should be some sort of trait/backstory/etc that disables caring and treating for other humans/humanoids but allows it for animals. I made a colonist who loves animals and wants to rescue them whenever they're hurt, but I gave her a trait that disables caring because I wanted her to be "weak" as far as performing/treating/caring for other colonists. Would this be hard to make?
AFAIK, yes, it would be difficult to implement. So far, the Doctor work type is used in treating colonists, prisoners, and animals alike. I may be wrong, but I believe to make a mod like this happen, the ability to treat animals would have to be moved from the Doctor work type to the Handle work type, or somewhere else.