Replace a type of vanilla mob

Hey Everyone!

I recently started learning about the random spawns function and was wondering If I could create a MythicMob that would always replace a certain vanilla mob when it spawns. For Example

I have a custom horse plugin on my server, so I'd rather vanilla horses didn't spawn but instead of just finding some 'AntiHorsePro' plugin I wanted to have some fun with it through MythicMobs...

I've created a player NPC wearing a horse costume with some funny dialog when you stand near him,...

Replace a type of vanilla mob