Mack is one of the many turtles that live in the pond ruled by Yertle the Turtle.
He is from the book Yertle the Turtle and Other Stories.
While the ungrateful King Yertle is on a stack of turtles to be king of things he sees, Mack (the turtle at the very bottom) tries to convince Yertle to stop all this staking of the turtles due to the worse of pains he and everyone else is feeling. Yertle does not listen to Mack but tells him to stay at the bottom and be quiet as he stacks more turtles on top of each other.
He burps after over hearing what Yertle is saying about collecting more. His Burp noises shook the turtle throne of the king and all the turtles were now free as Mack becomes new ruler of the turtles.