IN PATCH 3.2

You can buy it for 100g
In Un'Goro Crater
if you can survive its deadly poisen :O !

I think it's a quest.

Only HORDE can buy it.

The notes is here:
The ravasaur trainer Mor'vek has returned to Un'Goro and will offer to help Horde players raise and train a Venomhide Ravasaur as a mount... if they can survive the creature's deadly poison.

Someone lied to you my friend... this mount was from back when alli had wintersaber and horde had this, it was removed shortly after it was added due to alliance b**ching about how the horde could farm for the epic mount that didn't require epic training 10 levels earlier then the alliance since this was ungo and wintersabers was (is) Winterspring.

But now that epic mounts are trainable at 40 rather then 60 they are putting it back in since though still a 10 level earlier grind its long after everyone already has a epic mount.

Alight, I'm working on this one now with my Blood Elf, so this is what you have to do (as of Nov. 09)

The quest giver is a Troll named Mor'vek at 71,74 in Un'goro Crater.  First quest, you have to kill 20 or so ravasaurs.  Second, collect some eggs.  There might be another step I've forgotten, but this is the easy part.

Then he gives you the quest [They Grow Up So Fast] in which you "train" your mount.  You get a companion raptor that only works in Tanaris, Ungoro, and Silithus. The raptor will give you a series of 20 daily quests.  The daily quests are easy enough if you start around level 55 like I did, but be prepared for a grind.  After receiving 20 'baby teeth' from the raptor, return to Mor'vek with the teeth as well as 20 runecloth, 20 rugged leather (for the saddle) and 80 gold.

Then you get your ravasaur mount!