The myth of the Pegasi follows their patron deity, Pegasus.
In the beginning of the world, it was simply a large ball of dirt, devoid of life. Eventually, Pegasus descended onto the world, and was displeased at what he saw.
First, Pegasus took a chunk of earth and blew fire onto it. He threw this earth into the sky, and it became the sun. Once Pegasus saw the world in the full light, he was disgusted by its lack of flavour. He spat on the ground, creating water.
Pegasus was still unhappy. His water began to evaporate under the stare of the sun, so he took another chunk of earth and blew cool air onto it. He threw this earth into the sky, creating the moon.
Next, Pegasus looked around and became bored with the lack of variety in his world. He took and shaped earth, creating the trees. He breathed life onto the ground, creating the grass and flowers. Happy with the variety, but displeased by the stillness, Pegasus took more earth and fashioned it into many shapes. He breathed on these shapes, creating all of the animals of the world.
Pegasus was pleased for a while, but his world was static. Plants and animals began to suffer because his water could not reach them inland; the scorching rays of the sun were harmful, and Pegasus did not have the energy to move day into night constantly; his world needed care, and he could not tend to every plant and animal himself.
Pegasus raged in his frustration. His anger ravaged the world. Each time he stamped the ground the earth shaked and shifted, creating mountains and canyons. Each time he snorted the raging air killed vast swaths of earth, creating deserts. The very air chilled in fear to think of his coming, creating large, snowy regions inhospitable to life.
Eventually, seeing the destruction he caused, Pegasus calmed. In his calm, he finally came up with a solution. He took more earth, and formed it into the shapes of ponies. He breathed magic into them, and they became the unicorns. Pegasus instructed the unicorns in how to raise and lower the sun, and they set about the task.
Pegasus made more ponies, and breathed strength into them. They became the earth ponies. Pegasus instructed the ponies in how to tend crops and care for animals. They began to grow his crops and they led his animals to the water to drink.
Finally, Pegasus made his last batch of ponies, and into these he breathed flight. To them he gave the greatest power: the ability to control weather, and through it: nature. They set about bringing his water inland, giving it to the plants and animals as rain.
Finally pleased with his work, Pegasus left his creations, flying off towards the farthest end of his world. He settled down and took his rest, watching his world unfold and develop.