The live-bearing tooth carp, otherwise known as the mosquitofish or gambezi, is a small freshwater fish that's commonly introduced in mosquito-heavy areas to eat the larvae. It also demonstrates the ability to distinguish between group sizes - in other words, it can count. Up to four, at least. In an experiment conducted by the University of Padova in Italy, female mosquitofish being harassed by males were found to retreat to the largest group of other mosquitofish, up to four - it would always choose a group of four over a group of three. After four, its choice seemed to be random. That might seem simple, but most animals (and certainly most fish) have no numerical sense at all.
The finding is similar to one made while studying lions - lions can count the number of roars of unfamiliar lions, and react accordingly by sending more or fewer lions to investigate. A two-lion roar will call for a two- or three-lion investigating committee, while a three-lion roar will call for four.