Mounts can only be obtained during a Winterlight Solstice event.
You need a Percht Skull
to exchange for one mount. Dropped by The Percht Queen 
There are three variants of the sleigh and three colors each.
Variant 1 : Dark Percht Sleigh
Bright Percht Sleigh
Cold Percht Sleigh
Variant 2: Dark Percht Sleigh Variant
Bright Percht Sleigh Variant
Cold Percht Sleigh Variant
Variant 3 : Finished Dark Percht Sleigh
Finished Bright Percht Sleigh
Finished Cold Percht Sleigh
Mounts can be picked up after killing the queen. To get a mount you need to talk to NPC Frosty
NOTE :
Let me add that if you killed the queen once, you can only get one type of sleigh. For example: this year you killed the queen you can get all the colors from the first variant, if you kill the queen next year you can get 2 variant.