I have (among other things) a baby sunfish and a ram cichlid in the same tank, as I'm trying to grow out the sunfish in a warm environment, and their behavior is incredibly similar. Same swoop-and-pause method of exploring, same expressions of curiosity, unease, and aggression, same curiosity, even the same way of eating. Also, their fin arrangement is just about identical, and they both start out reasonably slender as babies and then get progressively taller-bodied as they grow up.
I know sunfish and cichlids are at least somewhat related, but are rams closer than the others? Because if I didn't know any better, and I saw a baby sunfish and a ram, I'd think one was some variant of the other.