A centaur that has a dolphin's or fish's tail in place of its rear legs. The front legs may be replaced with those of a lion. As you might expect, they are sea-beasts.
They often appear in Greek and Roman art, appearing at least as early as the first or second century AD. They are used to represent fertility.
http://www.eaudrey.com/myth/ichthyocentaur.htm has an example of an ichthyocentaur in Roman art.