Sirenia

Sirenia (commonly referred to as Sea cows) are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong (one species) and manatees (three species). Sirenia also includes Steller's sea cow, extinct since the 18th century, and a number of taxa known only from fossils. The order evolved during the Eocene, more than 50 million years ago.

Both, the dugong and the manatees, are believed to be the inspiration behind the Mermaid myth: where lonely deluded sailors saw the beasts as women with tails. Explained by Barney Stinson from How I Met Your Mother.