Among all the reindeer, two reindeers are missing: Vixen and Donner.
Vixen is replaced by a female fox, which are called vixen.
Donner is heavily implied to have been cooked into Kristmas kebab, as its examine text is A meaty donner kebab. The correct spelling is doner kebab, with the extra n being placed there purposely to imply Donner's unfortunate demise. This is reinforced by speaking to Dolph after relighting his nose, in which he exclaims I'd rather end up like Donner than have another sprite shoved where the sun don't shine!