rhinos of the oceans
I was charmed to read a while back that DNA analysis reveals that the closest living relative to the whale is the rhinoceros. The theory is that they had a common ancestor who lived in and around rivers, like the modern-day rhino. Some of them eventually made their way down to the sea, or were swept to the coast, and over time developed a tolerance for saltwater. They spent more and more time in the sea, until eventually it became their fulltime habitat.
Pretty cool, eh?