That's the bronze, silver and gold Yoshi's you get from completing the boss challenges. Which are hard - all the bosses have been turbocharged and are much nastier with their attacks, and there's no extra health. Except for Giant Bowser who's still a rather relaxing boss, especially compared to Baby Bowser who's just mean (the number of times I dodged into that boss instead of dodging away from its attacks...).
And yes, I used a Ganondorf Yoshi. It seemed fitting to bash the bosses with another boss :)