Kaiju
Over the next week, Wayvern was made to study royal magic and etiquette, along with some of Equestria's vast history. He wasn't mentioned in any of the thousand years since its founding.
"I couldn't find anything on you in history either," Twilight came in during one of his sessions. "And I own a copy of every book in Equestria." Wayvern thought it over. If he was the rightful king, why did Celestia share diplomatic power with Luna, Cadence, and more recently, Twilight.
"How did Celestia come to power?" Wayvern asked.
"What do you mean?"
"I mean, if I'm the rightful king, why is Celestia in charge?"
"You had other responsibilities, Wayvern," Celestia entered the room. "You had to leave those eleven Centuries ago to fulfill them. You were a Kaiju hunter."
"What's a Kaiju?" Twilight asked.
"Monsters," Wayvern said. "They could destroy entire worlds if they were allowed to… After Luna was born my parents were killed by Kaiju in the Line of duty, and I took their place as a Hunter."
"But there was one Kaiju that was never destroyed," Celestia Interrupted. "Nor would we want him destroyed. His name is Discord, he's an ally of Equestria now." A few days of studying later, and Twilight walked into his room to wake him up.
"Wayvern, wake up. It's time for your Coronation."