The Monster was invented by Madame Mousey with the help with the cats. The monster always appears Fievel's nightmares every night. Not in Fievel's dream, the monster catches the mice and put them in cages to be eaten by the felines which is the Madame's idea. The monster destroyed the barricade that protects from the cats. Fievel Mousekewitz and his friends destroyed (or killed) the machined monster, because of this Fievel no longer has a nightmare about Madame Mousey's horrifying monster. The monster is the secondary antagonist in An American Tail: The Mystery of the Night Monster.
Personality
Since the monster is a machine, the Madame made the monster sneaky, scary-looking and likes 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 of the monster is destroyed, it is really a machine.