I like how the game basically acknowledges that this isn't a real fusion either, since Rider Calyrex has TWO abilities instead of one.

It's like it's saying, "Yes, what you're facing has two abilities. Yes, it's basically just one Pokemon riding another one. Yes, this is more or less a two-on-one battle we've got going on here. No, it isn't fair. Deal with it."

Kyurem: Use mad science to create an incomplete abomination.
Necrozma: Allow it to devour the icon of the Sun or Moon in order to fuel it's immense power.
Calyrex: "I'm on a horse".