I think you might be thinking of a hippocamp.
http://en.wikipedia.org/wiki/Hippocamp
The hippocamp (ἱπποκαμπος, from ιππος meaning hors and κάμπος meaning monster). They were the the half horse, half fish creatures that drove the sea-chariot of the god of the sea Poseidon (who was also the god of horses).
Since the hippocamp is a half-horse half-fish creature, the tail is used for swimming.
There is no fantasy creature that is half horse half human and NOT called a centaur.
The Hippocampus is the genus name for seahorses. It is also the name of a mythological creature with the upper half of a horse and the lower half of a fish or dolphin. The hippocampus region of the brain was named due to its resemblance to a sea-horse.
The half horse, half eagle was a Hippogriff.
A centaur is a mythical creature that is half human, with the torso and head of a human, and half horse, with the body and legs of a horse. They are known for their strength, speed, and skill in archery.
centar
Buckbeak is a hippogriff (A magical half-horse half-eagle creature).
Half human, half fish is a Mermaid.
It is Centaurus.
It is Centaurus.
Centaurus is a Southern constellation representing a mythological creature that is half human and half horse. It is often depicted as a centaur holding a bow and arrow.