From the Super Mario Wiki
Slaughterforks are the result of forks that are forgotten and dropped under the tables of S.S. Caviar in Wario: Master of Disguise. These forks jump out of the table and try to attack Wario. Slaughterforks are about the size of Wario. They attack like Spoonatics and Jack the Knives.
Forks dropped under the table and forgotten are twisted into these monstrosities by their own bitter resentment.