The first lore topic I want to discuss is what Grogar’s role in ancient Equestria was.
In G4, his magic was immensely powerful. But this wasn't enough for him. Using an artifact called the Bewitching Bell, he stole magic from other sources. Ultimately, this led to his downfall. Gusty the Great used the Bell against him, stealing his magic, and then banished him.
We got to see the magic that was contained in the Bewitching Bell in Season 9. It powered two of the stronger villains far beyond Alicorn-level magic, and placed Cozy Glow on par with the Alicorns. At the time, the Bell was holding both Grogar's and Discord's magic.
This magic was instrumental in creating the monsters that live across Equestria today. It is said that he is responsible for the existence of many monsters that cause strife and hardship for the ponies. In G1, Grogar is even called "The Father of Monsters". created the minor monsters like the Ursas, the manticores, cockatrices, timberwolves, hydra, bunyips and chimeras. This was to spread chaos and misery, as this fueled his own personal magic.
My head cannon is this: According to mythology, the Dragons, Griffons, Kirin, Changelings, Hippogriff, Unicorns, and Pegasi are all monsters. Alicorns and Draconequis (Discord) have no real-world equivalents in mythology, but if we're putting Unicorns and Pegasi in the monster category, they also belong there. Basically, anything which does not appear in the real, natural world would be considered a monster. Grogar created all these creatures, endowing them with the ability to use magic. The fact that the Pegasi, Unicorns and Earth Ponies fought incessantly backs up this theory. They caused so much strife for themselves that Grogar just left them to their own devices.