I released version 1.5.8 yesterday afternoon, which includes the new Reindeer breed, as well as some bug fixes and extra configuration options. See the list of changes here. Download it here.
Reindeer spawn naturally in some cold and forest biomes. They can be tamed like horses, and if you name one after one of Santa's reindeer, it will gain the ability to fly. Its flying behavior is like that of a Nightmare - run fast and jump, and you'll be running on air. Once airborne, you can jump again to fly higher. However, due to their lack of flight maneuverability, if you turn too fast, your reindeer will float to the ground.
Reindeer can breed with all other types of magical horses. Baby reindeer are very cute.
Merry Christmas and Happy Holidays to you! :)
Here are some pictures of my reindeer, Prancer: