Rudolph

Rudolph is the first mount in Terraria, added in the 1.2.2 Christmas update.

Rudolph is spawned from the Reindeer Bells, and he is very quick. About double as fast as frostspark boots(needs confirmation).
You get the Reindeer Bells from the Frost Moon event, after wave 15(needs confirmation), where it will be dropable from the Ice Queen. It is not currently known if the Ice Queen can drop the item before wave 15, even though she spawns in the earlier waves as well.

Rudolph's speed stacks with the speed of running on asphalt, and therefore he can cover a LOT of ground very quickly.

As this is the first mount in Terraria, it may mean that there will be more in the 1.3 update.