The Night Monster, also known as the Manhattan Monster, is the the titular posthumous antagonist of An American Tail: The Mystery of the Night Monster. While thought of as a biological being, it is actually a robotic contraption invented by Madame Mousey with the help of her cat gang, fitted with ghastly flashing pictures of cat's face and a large buzz saw. The monster always haunt Fievel's nightmares every night throughout the film; in his dreams it appears as a fiery demonic cat with a mouse trap on its tongue. The rumor going around New York is that the monster catches the mice and devours them, but in actuality after they are captured by the robot the cats put them in cages to be sold to other cats and eaten, as part of the Madame's sinister plan. Every mouse sees the creature as something different; a Scottish mouse sees it as a lake monster, a Chinese mouse sees it as a dragon, and so on. Fievel is hired by The Daily Nibbler to draw each mouse's interpretation of the creature. Reed Daley sees belief in the monster as an opportunity to sell newspapers. Nellie Brie doesn't believe in the monster until it ambushes her and Fievel in an abandoned mansion, only to be defeated when Tony Toponi drops a chandelier on it.The monster then bypassed the barricade that protected the mice from the cats in their secluded neighborhood by traveling underground, and kidnapped nearly all of the mice including Fievel's family. Fievel Mousekewitz and his friends destroyed (or killed) the mechanical monster at the Daily Nibbler office, and after this Fievel no longer has nightmares about Madame Mousey's horrifying monster.
Since the monster is a machine, the Madame made the monster sneaky and scary-looking so as to terrorize the mice of New York City. The monster scares Fievel to death in his nightmares, which means he can scare mice children (7 or younger) easily. After the monster is destroyed, it is revealed as just a machine.
The monster's real strength is in the fear that it instills in the mice, who are fooled into thinking that it is a living creature.