Sharks

From howstuffworks "How Sharks Work" article:

We do know that sharks are solitary animals, for the most part. They typically live and hunt by themselves, joining up with other sharks only in certain circumstances, such as mating. Some sharks will form schools on occasion, however. Researchers aren't really sure why this occurs because sharks don't really need protection from predators and they don't feed in schools. At this point, it's still unclear why sharks behave this way. In any case, the occurrence is very rare. Most of the time, sharks swim alone.