Pebble

I remember having this guy as a quest companion earlier in Beta, already hoped he'd return as a permanent pet :)

Reward for Rock Lover (complete a daily quest to save this little fella from a cave 10 times)
The model of this pet is, of course, the same as the one of the NPC ;)

Unique Features
Pebble shares a model with the Frigid Frostling, so it is not a unique companion pet. This companion pet does not have any special animations, it simply shares the generic animations that other earth elementals have.

How to get it
Pebble will be automatically mailed to your character once you accomplish the achievement Rock Lover. To earn this achievement, you must first complete the daily quest Lost In The Deeps ten times. Obviously, this quest can be completed once per day, so it is a minimum of ten days to get this pet (the daily quest is not always offered, it is randomly available every day).

Source: Warcraft Vanity

This comes in an in-game mail once you get the achievement; but it doesn't mention him specifically. It's the generic 'You were supposed to receive an item but your bags were full' message. 'We found this in the Floating Nether and it seems to belong to you'.