in the beginning, there was nothing.
then, there was Harmony.

The god of ponykind is the first Alicorn, Harmony. Most contemporary pony religion posits that Harmony is older than time and the first living being. According to legend, she fashioned the planet of Ungulata and its sun, Solis, and moon, Selene, from stardust. Then, to populate the planet, she created beings in her image: the Alicorns.

However, Harmony was not the only cosmic being. From the nothingness of space, another being was born: Dissonance. The complete antithesis of Harmony, Dissonance saw Harmony's creation and was disappointed. The Alicorns were too perfect, untouched by hardship. So Dissonance sent them challenges. Dissonance created several godlings to rival the Alicorns - the Draconequus. At first, the Draconequus were simply mischievous and toyed with the Alicorns, but this eventually escalated into a catastrophic war. To end this battle, Harmony and Dissonance met each other in combat in the stars. This fight was bloody and long, and eventually ended with both of the gods' physical forms being completely destroyed as an explosion of pure magic sent them flying across the world. Their forms shattered, and where these pieces landed, powerful wellsprings of magic formed. The most notable of these are the Tree of Harmony and the Everfree Forest. The Tree of Harmony grew from a piece of Harmony's form, and the Everfree Forest grew from pieces of Dissonance's form.

These wellsprings of magic were not the only things that were altered by the explosive deaths of two gods. In the burst of energy created by the clash, the Alicorns and Draconequus were split apart as well. The Alicorns were split into three distinct beings: Unicorns, Earth Ponies, and Pegasi, while the Draconequus were split into several other creatures: the Dragons, the Changelings, the Centaurs, the Gargoyles, the Griffins, the Diamond Dogs, and most other sentient creatures in the world.

Of course, this is just legend fabricated by ponies in order to explain the different types of magic (Harmonic and Entropic), the creation of the world and the ponies, and the existence of magic itself. Every species has their own creation myths and explanations. Ponies believe themselves to be direct descendants of this first god and world-creator, who gave them their magic and cutie marks. They worship the spirit of Harmony and pray to her for help and aid, and powerfully magical ponies even alter themselves through powerful and ancient magic to emulate Harmony and her ideal creations. And as they worship and love the spirit of Harmony... to ponyfolk, the spirit of Dissonance is the antithesis to everything they stand for, which is why the Draconequus (being creatures of pure chaos, they spontaneously recreated themselves several times over the eons to terrorize the ponyfolk) are viewed historically with such revulsion.