Cuddles' Mouse is the pet of Cuddles in the episode Junk in the Trunk, where it's first seen riding on his shoulder. When Lifty and Shifty have Cuddles help them lift Lumpy's Elephant to put it in their van, they leave him to hold Lumpy's Elephant and simultaneously add his mouse to their stolen pet collection.
When Lumpy pursues Lifty and Shifty to retrieve his elephant, Lifty tries to throw various pets at Lumpy to slow him down. When he picks up the mouse to do the same, Lumpy's Elephant panics, due to its fear of mice. This causes it to bump into Shifty and make him lose control of the van. The mouse was among the many pets that Lumpy rescued and subsequently adopted, where it tries to eat the cookie Lumpy offered to his elephant. Due to its fear of mice, Lumpy's Elephant runs away and forces Lumpy to chase after it.