The clymene dolphin

Clymene dolphins. NOAA Southeast Fisheries Science Center

The clymene dolphin is a hybrid of two other species. All three species live in the Atlantic Ocean. This is the first known instance of hybridization among marine mammals leading to a new species, a phenomenon that is more commonly seen in plants, fish and birds. 

Ana R. Amaral, a researcher at the University of Lisbon and the American Museum of Natural History is the leading author of the study. Dr. Amaral said it was not clear exactly when in history the hybridization occurred - Scientist know that spinner and striped dolphins originated between one and three million years ago, so it was sometime after that.

http://www.nytimes.com/2014/01/14/science/scientists-find-rare-hybrid-of-two-other-dolphin-species.html?_r=0