Reindeer Bells

The Reindeer Bells is an item obtained from the Frost Moon event. It has a chance to drop from the Ice Queen during or after wave 15. The item is used to greatly improve a character's mobility and is, according to the official patch notes, a mount.

Use
When used, a buff is given to the character called Rudoplh (a misspelling in game meant to be Rudolph). Rudolph allows the character to run and fly faster than the maximum speed attainable from accessory items, and even stacks with them. Their flight height is increased by giving it a running start. If this is done, Rudolph can allow the character to reach the stratosphere in just one full rise. When the buff is active the character also shown riding a reindeer.

Limitations
Though the Rudolph buff will not prevent the player from using items (such as weapons or grappling hooks), doing so will dismount him/her and require Rudolph to be re-summoned. Nor is it possible to dash with the Tabi or Master Ninja Gear (though evading attacks can still occur). Rudolph also increases the height of the player, making it impossible to pass through doors normally.

Video
Example of Rudolph in action: YouTube: "How to get rudolph in terraria"