Madame Mousey is the main antagonist in An American Tail: The Mystery of the Night Monster, the fourth and final sequel to the classic American Tail. She is a miniature French poodle who has started living among the mice about this time, appearing at every crime scene where the Night Monster, Twitch has struck, claiming to be a fortune teller. She is also the mastermind behind the night monster all along. She is voiced by Candi Milo.
She faked being a soothsayer to get money from the mice. She created the Manhattan monster to get rid of the rodents because when she ran away from her owner, the other dogs didn't want to be around a dog who looks similar to a rodent. She snapped and got cats to help her make a monster to wipe away all the rodents of New York. She locked them in crates and sold them to cats to be eaten. In the end, Fievel Mousekewitz knocks her out by using a live wire to shock her. At the end of the fight with the cats, Tiger and Tony catch Mousey from escaping into the sewer, and the dog council sends her back to her owner. She claimed it was even worse than going to the city pound.
She despises Nellie for doubting the beliefs of the night monster and underestimating her. She gave Tony a fake clue to the night monster so Fievel and Nellie could go to an old house to be finished off.
- She is similar to Professor Ratigan from Disney's The Great Mouse Detective. They greatly hate being called a rat and lose their tempers when they are called one. However, unlike Ratigan, Madame Mousey has a bigger right since she is a french poodle, while Ratigan really is a rat.
- She normally speaks with a French accent, but switches to to a Brooklyn accent when angry, hinting that the French one is false.
- Mousey was considered to be a better villain than the villains in the third American Tail film. This is because Mr. Grasping, Toplofty, and O'Bloat has little to no personality traits to make them more interesting. Mousey on the other hand does which makes her much more loved by fans, but not as much as Cat R. Waul and Warren T. Rat.
- A small running gag in the movie is someone commenting on Mousey's size or that she looks like a rat and she starts growling, only to regain control a moment later.