SkullGreymon is a skeletal dragon digimon created by the Dark Spawn to serve as one of it's demon soldiers. It's mega form is that of BlackWarGreymon. It was a Digimon that put too much importance on fighting, and as a result of its combat instinct it clung to life despite its body rotting away, its whole body becoming nothing but bones. As it doesn't have even a scrap of intelligence to balance its combat instinct, its existence is a threat to other Digimon. SkullGreymon has not appeared yet in the series.
SkullGreymon is the result of Tai's reckless actions in trying to make Agumon digivolve to Ultimate. By overfeeding him and then deliberately putting himself into danger, he gets Greymon to digivolve but through Dark Digivolution into this horrible form. He annihilates the Greymon attacking them but doesn't differentiate between friend and foe. He only stops when he runs out of energy and reverts to Koromon. The Arrival of SkullGreymon This action breaks the confidence of both Tai and Agumon, which only recovers through Piximon's special training.The Piximon Cometh At the end of the series, Gennai reveals that SkullGreymon isn't a wrong digivolution for Agumon, just that it isn't a Digimon suited for their purposes, in order to quell the conversation about Virus Digimon. The Fate of Two Worlds