Blitzen was one of Santa Claus' reindeer.
While at the Nadir Motorway Service Station, a man dressed as Father Christmas approached the Eighth Doctor and Patricia Ryder and asked if anybody had seen his reindeer, one of which was Dasher. (AUDIO: Death in Blackpool)
Blitzen appeared in a shared dream, in which he pulled Santa's sleigh along with Rudolph and Donner. He was capable of flight. (TV: Last Christmas)