Who was a mythical monster kept in a labyrinth?

The Minotaur - a half-man and half-bull creature.

The King of Crete - Minos - confined it to a labyrinth created by Daedalus and fed it human sacrifices - young people from Athens sent as 'tribute' to Minos.

It was slain by Theseus with the help of Ariadne, daughter of Minos.