Colonel Mustard is the pet cat of Granny May. He appeared in Granny's Intuition when Granny May was taking a picture of him for a Christmas card.
In Granny's Pet Plan, he appears multiple times. One of his important roles was when he collapsed on Granny's remote, freeing the pets from their mind-controlled state.
In Don't Mess with the Best, he made a cameo when Granny May was giving him a bath. He was hypnotized by Victoria Best's recorder as well as Granny May.
He was seen with Granny May in Who's Your Granny?.