Lachleo

The Kyurem, Zekrom and Reshiram quests are inherently linked. You cannot get Zekrom and Reshiram without first doing Kyurem. The pokemon are linked and fuseable. Zekrom is not part of the christmas dungeon quest, you catch it in Eindoak along with Kyurem. However, you can still change it to xmas form via the spirit of christmas chair in Polar City. It would make total sense for the chair to also change the christmas form of Kyurem but player reasons for it not to are "because it's old". This doesn't really make any sense given the quests and pokemon are linked and the chair's existance is to change their form. Kyurem should also have it's christmas form available to be applied by the spirit of christmas chair in Polar City.