In myths, Pegasus is the immortal son of the sea god Poseidon and Medusa. However, as Medusa was cursed soon after, he and his brother Chrysaor remained inside her. After Medusa was slain by the hero Perseus, Pegasus sprang up from her decapitated neck stump, along with his brother Chrysaor.
Eventually, with the aid of Athena, the hero Bellerophon captured him. With his aid, Bellerophon was able to slay the Chimera by dropping lead into the Chimera's mouth which slowly suffocated the beast. In time, Bellerphon's burgeoning great pride drove him to attempt to fly to the top of Olympus. Zeus punished him for his arrogance by sending a gadfly to sting Pegasus, which made Pegasus buck off Bellerophon, permanently crippling him. Pegasus completed the trip to Mount Olympus on his own and took his place in Zeus' stables. He was often viewed as carrying Zeus' thunderbolts. In some stories, Zeus eventually placed him among the stars.
Pegasus' offspring, called Pegasi, is seen quite often. Pegasus is mentioned as the one true Pegasus up in the sky, flying around the world.
When sailing in the Mediterranean Sea, the crew of the Argo II is attacked by Chrysaor. He reveals himself to be the brother of Pegasus but hates his brother as he got all the attention. Percy also thinks about how this makes Pegasus his half-brother, making him related to all the Pegasi and Chrysaor.
After Coach Hedge contacted several wind-type spirits that they needed help, Pegasus himself and at least half a dozen other pegasi, including Percy's mount Blackjack, came to help. Pegasus helped transport the Athena Parthenos and allowed Reyna to ride him. Once they reached Camp Half-Blood and placed the Athena Parthenos on a yacht, Pegasus told Hedge that he should leave because his life force connects all pegasi, that he is immortal but his offspring are not. Pegasus then tells Reyna that he did not come to them in person because of Hedge's messages, but because he saw the friendship between Reyna and Scipio and gives Reyna the title "Horse Friend." He then flies off, after getting his pegasi to stay and help. Coach Hedge is stunned as Pegasus had not shown himself for centuries, yet he came for Reyna.
Pegasus is a massive stallion with a wingspan twice as wide as other pegasi. Pegasus's coat is a rich brown, mottled with red and gold around the muzzle - which Hedge claimed were marks where the stallion had emerged from the blood and ichor of his beheaded mother, Medusa. Pegasus's wings are the colors of an eagle's - gold, white, brown and rust - which made him look more handsome and regal. Reyna noted that he is the color of all pegasi, representing all of his offspring.
- Main article: Pegasus (species)
The pegasi are the children of Pegasus. They are all winged flying horses like him and are linked to him. As such, if Pegasus is injured, then all pegasi receive the same injury. Examples of Pegasus' children in the Percy Jackson and the Olympians series and The Heroes of Olympus series are:
- Blackjack ‒ a full black-coated Pegasus and is commonly used as Percy's main form of transportation.
- Guido ‒ a white Pegasus.
- Porkpie ‒ a white Pegasus.
- An unnamed Pegasus is shown in The Battle of the Labyrinth when Percy whistles four times after he, Annabeth, Grover, and Tyson got out of the Marriott hotel's basement with Rachel Elizabeth Dare and Nico di Angelo.
- Scipio ‒ a peanut butter colored Pegasus who was nicknamed Skippy and was owned by Reyna.
- According to some myths, Pegasus was given the task of carrying Zeus' lightning bolts.
- It is said that he flies around in the sky in the Sea of Monsters, but in Blood of Olympus, it is revealed that he hardly shows himself. This could specify that while he is constantly flying around, he is rarely seen.
- Reyna notes that Pegasus is the colors of all his offspring, yet black is not represented on Pegasus.