"Pony" is the general term used to refer to the various equines who inhabit the land of Equestria. There are four main varieties of pony:
Earth pony. These are similar to real-world horses, having no wings, horns, or obvious magical abilities. They are said to have a special connection with the land itself, and are generally believed to be the strongest and toughest of the breeds.
Pegasus ponies have wings and the ability to walk on clouds.
Alicorns are the fourth breed, and possess traits of the other three breeds combined: pegasus wings, unicorn horns (and magic), and earth pony strength. There are only two alicorns depicted in the show: Princess Celestia and her sister Princess Luna, who are near-divine in terms of both power and position.