The first time Celestia ever met Discord.

I see Discord as being substantially older than Celestia, at least by a half a century if not far more. Discord was also a creature born from pure spontaneity. No one knows where he came from, not even Discord himself. But he has been around since a time LONG before the founding of Equestria. Odds are he didn’t even grown up in the region that would become Equestria. It’s more likely he was born and lived further north before he met the alicorn sisters.
Because of his separation from pony-kind up until he was maybe 900-1100 years old (or the draconequus equivalent of like, fifteen) ponies were a completely new group of creatures he’d never seen before. He didn’t know how to speak and acted as any woodland animal would. He could chirp and bark and make an assortment of cat like noises. So when he stumbled upon Celestia and the not so bearded Starswirl, anything they said went over his head like Clayton talking to Tarzan.
Starswirl, at the time, couldn’t have been any older than sixteen or seventeen. Celestia was the pony equivalent of eight, but she never said how old she was was at the time.
Starswirl tried to shoo the mismatch beast away but Discord was captivated and enraptured with the small creatures he’d found. And they did magic. Discord was capable of fantastical shows of magic but didn’t posses the knowledge to use it.
Anyway, Discord followed the two back to castle. Starswirl tried to have Discord killed but Celestia pleaded they not hurt him. He wasn’t dangerous, just curious. Starswirl was reluctant but agreed. Discord was allowed to stay, so long as he was outdoors or under careful watch from Starswirl himself.

More about Starswirl and Discord to come later on.
Also I couldn’t resist giving Discord little juvenile spotting on his fur. He’ll grow out of it soon enough.