The tale of Discord is a sad and complicated one. Many eons ago, he was a simple eqqus stallion just like any other. He lorded over his mares in the spring and frolicked with his fellow stallions in the off-season. He, like many others, was caught in the Great Upheaval. He has hazy, dreamlike memories of the terror and fire of the destruction of his home. Where his story diverges from that of all others, is that he fell into the earth. The ground opened up beneath him and swallowed him whole, pulling him down into the heart of the world, and was bathed in the powerful magic found there. After a few short hours, the world spat him back up again, and save for a few scrapes and bruises, he was fine.
The real changes came after. Without a herd to follow, as they all left or died in the time Discord was missing, he found himself a cave to hide in and wait out the fires. In the dark, he began to change. He was struck with fevers and chills, his dark coat fell out in large clumps, he was struck with tremors and fainting, and he was constantly throwing up blood. His body was irradiated with magic, and he was all but dying. After the sickness came the pain. He was born anew limb-by-limb. He stretched and bent, his body turning into a horrifying amalgamation of different beasts. When it was all over, he was no longer Equus, but Draconequus.
With his new, horrifying body, came the most powerful magic in the world. He would blink in and out of the dimension, change the biochemical structures of all life he touched, and his thoughts and whims materialized as he thought of them. For a sick beast who did not even know how to read, this was a maddening and frightening time. Each equus he approached would run in fear or melt if he touched it. The other beasts of this horrifying liminal world would do the same. He was truly alone like no other creature had ever been.
After many lifetimes of this, he eventually began to control his powers, and use them almost exclusively for his own entertainment. Eventually, as all god-kings do, he got bored, and blinked out of this dimension for several millennia, waiting for things to get interesting again. When he returned, two magically gifted ponies were running the show. This threatened his perceived role as god of this world, and he had to put a stop to it. Revealing himself as Discord, god of chaos, and turned everything upside down and inside out, he was quite shocked and offended to be promptly turned into a statue and left in a back garden for nearly 1500 years.