Apparently the new Frostfell quest Merry Mischief from Candice Cheriweth by the stoves in the Frostfell Village has a tameable reindeer in it. It's running around in the outdoor portion of the instance, which can only be done once per year.
Advice from Feldon on the forums: "Be very careful taming the reindeer. There's just one of them, and the Cookie helper you get tends to kill it. Make sure the Cookie helper is on Passive and not defending you. If necessary, delete the quest, reset the zone, and get it again."