The White Cat is Gatomon. It is found in Highlight Haven in Dawn or can be degenerated from an Angewomon. As for Skullagumon, I think you mean BlackAgumon, which is a Dusk-only starting digimon, in the Process Factory(Dusk only), by degenerating a Candlemon and digivolving into BlackAgumon, or by degenerating a Vermilimon/Skullgreymon 2 stages.