The Rat Transformation Spell was a spell used to transform someone into a rat. It was famous for being used by Amy Madison twice: once on Buffy Summers while under a Love spell and once on herself to escape an angry mob.
The spell starts as energy that surrounds the caster than is cast on whomever it is pointed at. If the caster doesn't point at anyone, the caster gets affected by the spell themselves. While the caster can reverse the spell if it is cast on someone else, it is not possible for them to reverse it on themselves and needs a second party to be reversed.