Winterspring Frostsaber appeared when it goes to lv60. It is a tiger with a beautiful purple stripe. So how to obtain a Winterspring Frostsaber ?
Before CTM, you should be more patient for obtaining Winterspring Frostsaber. Because only 3 quests you can get. You need to keep doing these three quests. You can buy the mount and pets till getting Exalted Reputation. 
In the 4.1 Patch, Blizard has changed a much easier approach to get the mount. The main quest will ask you to raise a Winterspring Cub. Every day your Winterspring Cub will give you one of a few different daily quests. When completed you will receive a Winterspring Cub Whisker. After having collected 20 cub whiskers you will get Winterspring Frostsaber. 
Winterspring Frostsaber is the alliance counterpart mount, just same as Venomhide Ravasaur for horde only. Both mounts will change if you transfer faction between alliance and horde.
Come and buy such beautiful and cheap mount!