Yoshi is the main animal friend that aids Mario in his quest to thwart Bowser.
Yoshi is one of Mario's best friends since Super Mario World where he saved him in the first level of the game right after Yoshi's House. He was trapped in a egg and stuffed into a block by Bowser in order to prevent him from rescuing his friends from the Koopaling's towers. They've met before in Yoshi's Island, where he saved Mario's life by transporting him to the stork that will give the baby to his future parents. Like in Super Mario World Yoshi is a very useful power up, but in the hack he isn't exactly happy of the idea of help Mario.
After Mario's condemnent from the Mushroom Kingdom the ex-king aims to conquer Bowser's Kingdom. Yoshi, aware of the tyrant's arrival, decides to run away from home and leaves a note to Mario asking to stay away from him. Despite his efforts the plumber still manages to catch him and forces the dinosaur to help him in his quest. Despite the initial diffidence with Mario, he then determines himself to aid the ex-king, starting with saving him from a Hungry Boo.