The legend of the Tooth Fairy is about a fairy that gives a child money and gifts in exchange for a baby tooth that has fallen out. Children typically place the tooth under their pillow at night. The fairy is said to take the tooth from under the pillow and replace it with money once they have fallen asleep.
In "Prick Up Your Ears", when Stewie Griffin learns of this after losing a tooth, he mistakes the Tooth Fairy for "a tooth thief". When his dog, Brian, sees how much this is affecting Stewie, he tells him the Tooth Fairy isn't real, but then they see Stewie's tooth missing from under the pillow. Sure enough, the Tooth Fairy took it and is in reality a perverted man, that is sexually attracted to teeth.