It requires a badge and a technique. Don't ask me why it is. It just is.

[ But for real though. Corviknight is loving this sort of attention. It's a bit harder to get now that he's so massive. When it had been a small thing, he would bother Bakugou all the time in the middle of the night. ]

Anyway. You only get that one. The rest you have to catch in the wild or you just get eggs.

[ Which means he's releasing his other four. Houndoom, Sandslash, Chandelure...

And an Alolan Ninetales. ]