The Mutant Muck Monster is the toy Arthur gets from Grandma Thora in "Arthur's Toy Trouble." This toy was bought at David's Toys, and was found in the bargain bin, due to being a factory defect. The cost of this toy was $6.47, but Freddy K. said it would only be worth $5 after 10 years.
The monster's head is human, even though humans are rarely seen on the show. Arthur describes the head as "kind of normal".
A Muck Monster with the same defect is seen in "Citizen Cheikh", suggesting that the defect is not very rare.