Sea cows are the common name for sirenians—the plant-eating marine mammals that include manatees and dugongs. People most often use “sea cow” for manatees, but dugongs belong to the same group.
Sirenians are fully aquatic mammals that graze on seagrasses and other aquatic plants. Manatees have a broad, paddle-shaped tail; dugongs have a fluked tail more like a whale’s. A related species, Steller’s sea cow, was hunted to extinction in the 1700s.
Source: Natural History Museum — Manatees: what is a sea cow?