Blizzard have snuck this little battle pet in recently (or at least, it has only been found recently!) .. you can find it in an underwater cave in Borean Tundra. It hatches from an egg with the purple "treasure" glow and is a virtually instant respawn .. if you can find it in the crowd of people stood on it 
There is some good advice on the wowhead forum, including:
You can find the White Murloc Egg which give the pet in the borean Tundra, in a marine cave at the Riplash Ruins. Coord of the Cave are 56 - 88. (you can see several screenshots on the egg page to get to the cave or check http://imgur.com/a/GJYoD)
The entrance is really tricky to find.
When you are at the coord, you can see in front of the entrance
- A colone
- 2 red Seaweed
- severals green Seaweed
- 2 purple Seaweed when you go further.
- you have to go further and up to get in the cave.
The egg in on the ground, behind the altar.
