Rimefang

Rimefang is a frost wyrm of the Frostbrood. Rimefang was originally the frost wyrm mount of Scourgelord Tyrannus the dreadful master of the Pit of Saron, who was killed by the champions of Jaina Proudmoore and Sylvanas Windrunner. After fleeing the scene, Rimefang waited as a guardian ready to kill the Ashen Verdict's heroes invading the Frostwing Halls.

Frosty was accidentally created when the Lich King's deathly powers raised Rimefang from undeath.

Rimefang's harness, left over after his defeat, had the power to bind frost wyrms to others' will. Vrykul raiders used it in the Pit of Saron for exactly that. After the Knights of the Ebon Blade retrieved the harness, they used it to raise Thorignir drakes at Thorim's Peak in Stormheim.

Pit of Saron
In the Pit of Saron he serves as mount to Scourgelord Tyrannus. During the Tyrannus encounter he flies overhead raining down ice upon the players, and flies away when Tyrannus is slain.

Icecrown Citadel: Frostwing Halls
Rimefang also appears in the Frostwing Halls inside the Icecrown Citadel raid. Upon killing him and Spinestalker, the queen of the Frostbrood, Sindragosa, will spawn.