Mud Boy is the eponymous character from the episode Mud Boy. He was originally a drawing Peter made in the mud that Jumanji brought to life, as an allegory of Peter's youthful fun-loving nature and tendency to disregard rules from his sister Judy.
Because Peter wished to have his own little brother, who likes him, and always wants to play and have fun, Mud Boy is essentially an allegory of Peter himself, being that he is somewhat insane, hyperactive, enjoys playing with mud, and above all else likes to have fun, but it comes at a cost of disregard for supervision and the safety of other people, making him hate sisters like Judy, who upsets him after he tips the canoe over and refers to her a a boss monster. He soon becomes insufferable and doesn't take authority from anyone but Peter or rejection well. Conversely, his mud sister is the one that stops his rampage (who also serves as an allegory to Judy and also voiced by Debi Derribery).
After hearing Peter and Judy's clue, Alan figures it could mean the old Dirt Mines on the other side of the Bayou and heads off to find the paddles for his canoe. Peter draws a creature in the mud, which comes to life and Peter names it Mud Boy. Judy isn't too happy when she finds out about it, but they take it with them. In the canoe, Mud Boy fools around and scares Alan causing the canoe to tip over. An alligator destroys the canoe although they get safely to shore. Judy tries to impose rules on Mud Boy, but Mud Boy, who doesn't like rules, yells at her and runs off.
At the dirt mines a machine attacks them and Peter gets separated from Judy and Alan and encounters Mud Boy in an underground tunnel and the two head off to find Judy and Alan. Mud Boy reveals that he just wants to have fun and takes Peter to a place with some spongy substance and a nearby mud waterfall. The two bounce around, and then Mud Boy decides to get rid of Alan and Judy so he and imprisons Peter can have fun forever and takes off to get them after sticking Peter to a wall. Alan and Judy are nearly drowned in the mud river while Mud Boy just watches, but Peter manages to free himself and saves them. Mud Boy adds more mud to himself and grows to giant size and chases the three. Peter finally stops him by creating Mud Girl, Mud Boy's older sister and Mud Girl orders Mud Boy around like Judy does to Peter and gets him to stop. Mud Boy returns to normal and heads off with Mud Girl, but not before he gives Peter a friendly wave and appreciates what Peter put on Mud Girl for him: a kick me sign inscribed into her back. Alan realizes that Mud Boy was the "ruin from the earth" and Mud Girl was the "magic birth" thus solving Peter's clue and sending him and Judy back home.