The Grim Adventures of Billy & Mandy

Attack of the Clowns (3x26)

:

After Billy wakes up from another nightmare involving clowns, he is more terrified than ever towards them, so Mandy and Grim try to cure him of this clown-a-phobia. First, Mandy asks Grim to bring to life a clown's head made on paper mache to entertain Billy, but the spell apparently doesn't work. Then, they dress up as clowns in an attempt to look harmless. However, after consulting with his imaginary "Inner Frat Boy", Billy changes his feeling from being afraid to being angry at clowns, and attacks them with Grim's scythe. In the end, Billy is apparently cured of his fear, but then the paper mache clown attacks and eats them.