SkullGreymon is an alternate Ultimate form of Agumon.
The "skull" prefix in his name refers to his body, which is completely fleshless and entirely composed of bone. He has a missile that lies on his back that is shaped like a shark.
SkullGreymon is a destructive and uncontrollable Digimon that results when Greymon is induced to Digivolve incorrectly.
In The Arrival of SkullGreymon,
Digimon Adventure 02
In The Emperor's New Home,
In The Captive Digimon,