Toads are small amphibians that live in swamp-like areas. A common conception for "evil witches" is that they turn innocent people into toads.
When the Headless Horseman severed Piper Halliwell's head, Slick got snippy with her considering she didn't have her hands to freeze him. "Ah-ah-ah... I still got a mouth, I can turn you into a toad." Clearly he didn't heed her warning, as he had been turned into a toad minutes later. She then noted, "Anybody else wanna try me?".