"Little Bunny Foo Foo" is a folk song with unknown origins. It was performed by Butters Stotch in the Season Four episode, "Something You Can Do with Your Finger".
Background
Eric Cartman held auditions in his living room looking for a fifth member for his boy band, Fingerbang. Butters sings this song, for his audition, however, he does not win.
A boy resembling Schroeder, from the Peanuts comic strip, supplies the music accompaniment for auditions.
Lyrics
Little bunny Foo Foo
Hopping through the forest
Scooping up the field mice
And bopping them on the head
Down came a white angel and she said
"Little bunny Foo Foo
I don't want to see you
Scooping up the field mice
And bopping them on the head."
So now I'm gonna turn you into a worm, mbuh, mBunny Foo-Foo... Eh, beh-POOF
Little Wormy Foo-Foo crawlin' through the forest
Gettin scooped up by the field mice who mah-m then they bopped 'im on the head