Mr. Fox (or sometimes Foxy) is a character who appeared in the original shorts and the 1972 series. He is a sly fox who is almost always doing bad things but is still usually fairly friendly with Calimero despite his naivety getting Mr. Fox in trouble most of the time.
Some of Mr. Fox's tricks include:
Tricking Calimero into giving him and his friends free drinks (in Calimero and the Rascals)
Selling fake medicine (in Calimero the Actor); Calimero tried to help but ended up exposing Mr. Fox's ruse
Working with the Wolf to cheat in a go-kart race (in Books and Motors); he was foiled by Calimero who threw the nails in front of his car
Using Calimero as a singer but taking all the money earned for himself
Helping Calimero take a man's picture for him but stealing the man's wallet when he wasn't looking; the photo Calimero took caught Mr. Fox in the act