A strange race, calling themselves Orcs, appears. They are tribal, brutal, and violent in nature.
They raid the Gryphon Kingdom continually for some time, moving from place to place, taking what they want or need from villages.
The King has had enough, and with a hasty alliance with Minotaurs aiding him, he traps the tribe in a pass and slaughters them.
Celestia arrives too late to stop him, but finds a survivor amongst the dead.
Feeling guilt over not being able to save him, Celestia takes the poor soul back with her to Equestria, and hopes the Gryphon King is wrong about how his people are.

Most if not all sentient races in Equestria will be depicted as Anthropomorphic
So this is an honestly very strange idea that came to me. I'm not sure how many 'Orc in Equestria' stories there are, but damn it, I'm going to give it a shot.