Let's get some things ironed out about the famed flying horse.
First, he's a stallion - not a mare as some people try to argue.
He's almost always depicted as white.
In most, and the earliest, stories, he and his brother, Chrysaor, were sired by Poseidon and birthed by the Gorgon, Medeusa - they were born from her blood either as Perseus cut off her head or when droplets of her blood fell into the sea.
His brother is not a horse. Chrysaor is always depicted as a young man.
Okay, have I left anything out?
Now that that's been cleared up, let's talk about other winged horses. Considering the facts above, should they, in any good conscience, be referred to as pegasi?