Pegasus Ponies are ponies that are born with wings, and the magical ability to walk on clouds.
Sporting wings, pegasus ponies (pegasi) have the naturally occurring magical ability to walk on clouds, and have an affinity with the air (Unicorns can learn a spell that allows other types of ponies to walk on clouds too, and to grant gauze-like butterfly wings, though these are temporary and verry sensitive). This also gives them the ability to physically manipulate weather via introducing or removing clouds from an area, creating rainbows, manufacturing snowflakes, and other weather-related effects. With this ability, they can also create buildings out of clouds, such as Cloudsdale, Rainbow Dash's house, and the ancient pegasus tribe's cloud homeland.
Although Equestria boasts varying levels of technology, such as motorised scooters, building equipment, horse drawn carriages, etc., Pegasus ponies are the only breed of pony to be shown working in factory conditions.
Pegasi are found in many places in Equestria, but Cloudsdale is the only known all-pegasi location, given its being entirely made of clouds, and many miles above ground. Most Pegasi enjoy their time spent in the sky, but some, like Fluttershy, prefer their hooves on the ground.
Many centuries before modern day-Equestria in the show, pegasi, unicorns, and earth ponies lived divided. The pegasus tribe lived in a cloud land high over the earth, and were a noteably military race, with many statues and murals depicting soldiers and military actions. Their architecture, which featured many pillars and columns, bore a distinctly Greek appearance, and their armour and weapons were highly reminiscent of greek hoplites, including plumed helmets and doru (spears carried by phalanx formations). It is implied that the pegasi lived under a military oligarchy, or possibly a dictatorship, given that they were led by Commander Hurricane.
The pegasus tribe maintained a strained and mistrustful alliance with the earth ponies and unicorns. In return for food from the earth ponies and the raising of the sun and moon by the unicorns, the pegasi stewerded the weather, maintaining the seasons. Unfortunately, the mutual loathing between the tribes attracted malevolent nature spirits known as Windegoes (based on the native American Wendegoes) who brought about a never ending blizzard. As conflict began to erupt between the tribes, the blizzard became worse, eventually leading to a meeting between the three societies. Following the unsuccessful meeting, the pegasi decide to leave their dying land in search of a new one, and the leaders and their proteges make their way to what would one day become Equestria. Unfortunately, the other two tribes get there as well, and in a short space of time, the blizzard follows them as they begin to argue.
The ponies were trapped within a cave to take shelter from the blizzard, but as they continue to argue, the leaders are all frozen solid, leaving their second-in-commands to huddle together. Clover the Clever, the unicorn lieutenant, realises that the blizzard is being caused by their hatred towards each other feeding the windigoes. In the final moments, the ponies realise that they actually rather like each other, and become friends, which ignites the magical fire of friendship that banishes the windigoes, and begins melting the damage of the blizzard.
The pegasi agree to unite with their brethren ponies, and together, they founded Equestria.