Butt Stallion

Eminence — When Butt Stallion is on the battlefield or the command zone, all horse creatures are mounts in addition to their other types. Saddle power must be {1} or greater. If mounted by another horse creature create a 3/3 Rainbow horse creature token."
Butt Stallion gets a +1/+1 for each horse on the battlefield and a +1/+0 for each time a mount was saddled.