Volvagia

The subterranean lava dragon is a terrifying beast which threatens the inhabitants of death mountain... or so it seems. Link first encounters Volvagia when they were both still children. He bought the young dragon from a merchant and decides to keep it as a pet. After some initial antipathy they became friends. During the time Link was sealed in the sacred realm, Ganondorf brainwashed Volvagia and transformed it into a mindless monster in order to spread terror across Hyrule. Link and Volvagia eventually ended up fighting each other and Link was forced to kill his former friend.