Stellar's sea cow
Related to the manatee and the dugong, this plump sea dweller once lived in the Arctic waters of the North Pacific.
When they were first discovered, sea cows already had a limited range, and their slow swimming speed and gentle nature made them easy targets for hunters. Within 27 years of their discovery by Europeans, they were declared extinct in 1768.
Because of the frigid waters where they lived, Stellar's sea cows grew to immense sizes, with reports putting them at around 30 feet long and weighing up to 10 tons. Unfortunately, it's their size and their fat which made them such valuable commodities for early Alaskan settlers and explorers.