Aegislash along with its prevolutions is the only Pokémon with steel/ghost unique matchup.
Aegislash and Tyrantrum are both Pokemon designed by Hitoshi Ariga. While the two Pokemon are entirely different, they share a shiny color-scheme that references the Mega Man Series. Shiny Aegislash shares the same colors as Rogue, while Shiny Tyrantrum shares the same colors as Mega Man. Both from the Star Force series. This could potentially be a referene to the fact that Rogue has a posessed sword.