Grogar were a grand wizard from a far away place. Ponies of the land that later is get to be known as Equestria welcomed the ram to their community, giving him a place where he can practice his magic. Grogar happily helped the ponies with his powers. Something was off about the ram’s magic, but the ponies didn’t question it. If he helps them and does good with it can’t be evil magic. They even let one of their own, a young filly to become his student.

However, there was more to Grogar intentions, just like there was more to him leaving his homeland. As in, his smile hid an evil scheme. Using the animals of the land to morph them into monsters in his experiments. Originally he wanted to use the animals, but Grogar couldn’t pass on such powerful and interesting creatures like the ponies.

When ponies started to disappear and monster started to terrorize them, is when the others caught on and found out the truth. They wanted banished Grogar, but he didn’t take it. Not so happily, loosing such a great source of a subject, cursed the ponies with his magic, destroying them all. Well all, expect for one. His ex-student, Gusty.

She was the only one powerful enough (and with knowledge of Grogar’s magic) who could survived. Later she managed to out smart the ram and steal the major source of his powers, The Bewitching Bell. And she knew how to use it, all thanks to being Grogar’s student. With it and her own magic Gusty managed to do what her fellow ponies couldn’t, banish Grogar.

However, she could never know that Grogar may able to return one day.

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Discord’s Grogar disguise: Not 100%, but considering it been thousands of years and Discord created it from memory it’s pretty close. He may also put his mental image on it, thinking Grogar is an old, weak man. Some his own features came through, such Discord’s eyes and how his own magic look like and works.

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Extra infos:

• Gusty and the ponies were a unicorn race that’s not around anymore. They weren’t part of THE unicorn tribe. In fact they lived houndreds of years before the three tribes found the soon to be Equestria.
• Grogar the reason why monsters exist. Hence the name Father of all Monsters.
• Grogar was the Emperor of Ancient Equestria, but only for short bit. Maybe a few months.
• It’s yet to be known where Grogar came from. He did say he used a portal, and his homeland also had creatures similar to ponies.
• Why did Grogar take on Gusty as a student? Probably multiple reasons. Hubris, that he can do what he wants under the nose of one of the ponies. So he could analyze them up close. Thinking he could have a pony servent. Or maybe he saw something in Gusty.