Pegasus

A Pegasus was an immortal winged horse, one of the two children of Poseidon and Medusa. Along with his brother, the golden-sworded Chrysaor, Pegasus sprang forth most miraculously from his pregnant mother’s neck after Perseus had beheaded her. He belonged to the Greek hero Bellerophon for some time, before being stabled by Zeus and tasked with drawing the chariot of his thunderbolts.

Monster High
Not much is known about them, as the only known Pagasus is a hybrid. Pyxis Prepstockings is a Fright-Mare who inhabits the Dream Pastures, a secret place within the walls of the Monster High Catacombs, a place where the monster's dream collide. She was born when a pegasus once dreamt and this dream collided with a Nightmare from the Dream Pastures, creating the unique being, Pyxis.

It's most likely in monster high there are many pegasus monsters, unlike the original myth where there is only one.

Description
Since the only pegasus is a hybrid, it's unknown what they would look like in monster high exactly. But Pyxis Prepstockings is shown to have white wings on her back. Her lower centaur like half is due to the fact that she is a pegasus/centaur hybrid.

So it's unclear if a pegasus is similar to a centaur or not.

Known Pegasus

 * Pyxis Prepstockings