Pezosiren is a mammal confirmed to appear in Additional Creatures: Endemics.
Pezosiren, meaning "Walking Sirenian", is a very early sirenian species that lived in Jamaica during the Eocene Epoch. It still had fully functional limbs, but the structure of its jaws and teeth indicated that, like modern manatees and dugongs it is the ancestor of, it fed on marine plants. It is believed it was a semi-aquatic creature that lived in and around the saltwater lagoons that it called home, swimming around in a similar manner to a hippo.
It is unknown exactly what role Pezosiren will play in Additional Creatures. It is mostly just confirmed, although the developer has shown interest in making it "sniff out" rare resources, much like a truffle pig. The design also shows it with algae growing on its back, like with what happens with many modern sirenians.