The Sei whale (Balaenoptera borealis) was an aquatic mammal indigenous to Earth. It was a cetacean species. It was a cosmopolitan species native to most of Earth's oceans, save the very northern ones.
In 1986, the Cetacean Institute had a display with models and information thereon of different whale species, among them a sei whale. It was the second biggest whale species depicted, second only to the blue whale. (Star Trek IV: The Voyage Home)