Note to self: A Yoshi boss. To begin the fight you free it from an egg by jumping on the egg. It then should display Level Message 1 for that level (preferably you should be able to turn this off). Yoshi will then run around the room, occasionally transforming into the different Yoshi colours: Red will allow him to pause and shoot fireballs at you while running around the room. Blue will allow him to fly up into the sky and attempt to divebomb you the way Boom Boom does. Yellow Yoshi will jump around the room, following you as you go, creating sand clouds that will damage you the same way the sand clouds normally do.
In order to damage this boss, you need to land on its' back either while it has paused to shoot a fireball, while it is divebombing, or while it is running around as an ordinary green Yoshi, and then punch it in the back of the head. The amount of times you need to punch it to defeat it should be customizable. Every time it is hit in the back of the head, it should play the "Yoshi hurt" sound effect. Once it has been hit enough to be defeated, it should fall off the screen and Mario should automatically dismount it.