Ichtyocentaurs

In Greek mythology, ichtyocentaurs were sea-dwelling centaurs.

Description
Ichtyocentaurs have the upper torso of a human, front legs of a horse and a long tail of a fish. They often bear peculiar horns that resemble a crustacean appendage ending with a pincer.

It's vague if there ever was more than two ichtyocentaurs. Most depictions in art refer only to two sea-centaurs, brothers Aphros and Bythos, but there are many artworks where context is not clear and there are no labels. Medieval bestiary Monstorum Historia (1642) has a female ichtyocentaur which quite obviously isn't the brother of the other ichtyocentaur portrayed in the book. They also have webbed feet instead of decidedly horse hooves, which are a standard in Ancient Greek art. Mythological literature of the same period of time also has no mentions other than Aphros and Bythos.

Origin
There are two versions of the origins of Ichtyocentaurs. One version has them an offspring of a union between Cronus and Philyra. This seems unlikely, as Philyra was transformed into a linden tree because she felt ashamed for bearing Chiron.

Another version has Poseidon and Amphitrite as their parents, which appears as a more plausible origin myth. They certainly are linked to Poseidon, as they can often be seen swimming on both sides of Poseidon's chariot. This is supported even more by the fact that Poseidon is also the god of horses.

Aphros may have been the foster father of Aphrodite, and the two brothers brough fully grown Aphrodite to the shore. This pleased Zeus so much that he turned Aphros and Bythos to constellation of Pisces.

The two named sea-centaurs were referred as demigods, and as such aided their parents in governing the sea. As a species, however, it is unknown how they came into being. Purely speculating, of course Poseidon and Amphitrite may have had a female ichtyocentaur as a child at one point another, which could have been the dawn of a new species.