"OOF OOF OOOOF! OOF OOF OOOOOF OOOOOOOOOOOF! I AM THE ONLY OOF OF ME. BABY THAT'S THE PETER GRIFFIN IN ME!"
Leafeon heard this random pokemon singing. She decided to investigate.
It was on a mountain.
She came to the top of the mountain.

It was a Glaceon showering.

DUN DUN DUUUUNNNN

Glaceon looked over at our main character. "Oh hey. Wanna be sisters?"
Leafeon replied with; "Yeah sure."

And that's how Glaceon was in the same generation as Leafeon.