Steller's sea cow was the giant marine mammal, related to the dugong, that humans hunted to extinction around the Commander Islands in the 18th century. Its scientific name was Hydrodamalis gigas, and it belonged to the order Sirenia, which includes living dugongs and manatees. It was not a cow, but its grazing lifestyle and massive body inspired the name.
Naturalist Georg Wilhelm Steller described the animal after encountering it during Vitus Bering's Great Northern Expedition in 1741. Adults may have reached roughly 9 meters in length and weighed several tons. They fed mainly on kelp in shallow coastal waters and had a thick layer of blubber, useful in the cold Bering Sea. Instead of true teeth, they had bristly mouth structures and keratinous plates for processing vegetation.
Within about 27 years of European discovery, fur traders and sailors had hunted the remaining population for meat, fat, and hide. The species was probably already restricted and vulnerable before that pressure. Its rapid disappearance is a classic example of how a small, isolated population can be driven extinct soon after outside contact.