Perserved Holly makes your mount turn into a reindeer for about a hour (I know im a little late on this, But I saved mine and found this today)
What you got to to, is use Perserved Holly (If you still have any from the X-mas event) and go into a random heroic. WITH your mount as a reindeer. Your hour on perserved holly MUST clear during the instance. so withing the last 15 miniutes of the Perserved Holly, Just go into a heroic.
Then once your finished, leave ! and thats it. You still have the mount.
Perserved Holly - http://www.wowhead.com/?item=21213
Screenshot:
You can see how I just left the heroic, and I dont have Perserved holly anymore, Just the mount.