Here's the jist on this guy.
Another critter needed for the achievement added in 3.08 Pest Control (at the time of this post, it's not in database).
Horde:
Head to Grom'gol, then head up the coast as if you're going to The Deadmines or to raid Westfall. Spam your target macro once you leave the beach area, and you'll find them no problem. Found one halfway between the beach and The Deadmines summoning stone.
Alliance:
Head to Sentinel Hill, then make your way to The Deadmines stone, you should pick one up before you even reach it. If not just scout the nearby area / farmsteads.

Helpful macro for the achievement:

• /tar https://vanillawowdb.com/?npc=3300 Adder
• /tar https://vanillawowdb.com/?npc=24270 Devouring Maggot
• /tar https://vanillawowdb.com/?npc=24174 Fjord Rat
• /tar https://vanillawowdb.com/?npc=16068 Larva
• /tar https://vanillawowdb.com/?npc=4953 Moccasin
• /tar https://vanillawowdb.com/?npc=4075 Rat
• /tar https://vanillawowdb.com/?npc=15476 Scorpion
• /tar https://vanillawowdb.com/?npc=14881 Spider
• /tar https://vanillawowdb.com/?npc=28202 Zul'Drak Rat
• /tar https://vanillawowdb.com/?npc=32261 Crystal Spider
• /tar https://vanillawowdb.com/?npc=9699 Fire Beetle
• /tar https://vanillawowdb.com/?npc=32258 Gold Beetle
• /tar https://vanillawowdb.com/?npc=16030 Maggot
• /tar https://vanillawowdb.com/?npc=6271 Mouse
• /tar https://vanillawowdb.com/?npc=4076 Roach
• /tar https://vanillawowdb.com/?npc=2914 Snake
• /tar https://vanillawowdb.com/?npc=32428 Underbelly Rat

Helpful macro for finding the Larva:
/tar Larva
/stopmacro [noexists]
/tell (YourCharacterName) > %t <

Remember to replace YourCharacterName with, well Your Character's Name.

Going to repost this on the others.

Found more mice in Grizzly Hills in the patch of land between Venture Bay and Conquest Hold around 19, 74. They are part of a patrolling group and not hard to find.

Coyotes eat the mice, so be sure to kill any you see while making your rounds to up your chances.

Important:

The critters of Westfall share their spawn location. So if you do not find any mouse around the locations on the map, kill all the Snakes and Prairie Dogs and the mouse will spawn.

Took me about an hour to find out........ Two minutes later I got mine.

It took me about a half an hour to no avil. After I got a plan it took me 5 min.. Here's what I did.

I kept the above map up, and I started at Sentinel Hill, killing the snakes and prairie dogs(pd), then I moved up to the pass Southeast of Moonbrook. I killed the Snakes and pds. Then to the area behind the inn with the Defias, again killing Snakes and pds. Back to Sentinel Hill, then to the Pass. I found my mouse in the Pass.

Confirmed to be around 19, 74 in Grizzly Hills

As above they do exist in Grizzly Hills just north of Venture bay.

Rather than using /tar macros and all that jazz just press V and look for packs of critters, should be nice and clear if there is a mouse hiding in with them. Then pick your favorite AoE attack and perform some critter slaughter. Safe in the knowledge you're one less rodent away from another achievement.

You can indeed find this critter in Grizzly Hills just north east of Venture bay (right below Conquest Hold) They will spawn along with other critters from the patch of cut down trees and run north then despawn. If you sit just north of the cut trees in the small clearing you'll have four or five packs run by. simple aoe and you got em.

https://vanillawowdb.com/?maps=394:196741185757203755209741186735193765 https://vanillawowdb.com/?maps=394:196741185757203755209741186735193765

Map of the location for those who hate going by cords.

If you don't see any right away just site tight, they seem to spawn randomly and will always coem from the patch of cut down trees and run north.

So if your high level and don't feel like going to westfall for this, heres a better place for it :3

I think there is something wrong with the mice in Northern Mulgore after the Shattering...

http://i.imgur.com/zJIVD.jpg http://i.imgur.com/zJIVD.jpg

If you havent seen them, the rodent wars in the northwest corner of Mulgore are worth the trip. Perhaps the most outlandish easter egg yet, the Prairie Dogs, Rabbits, and Mice in the area periodically grow larger than Gnomes, take up weapons and kill their rivals.

The Prairie Dogs don helmets and daggers. The Rabbits brandish battle axes, and the Mice dual-wield shotguns. If you get there and dont see anything, be patient. You'll soon be rewarded!

Possibly a Princess Bride R.O.U.S. (Rodents Of unusual Size) reference?

They're preparing for war!

If you see one running around in a little red cape holding a big red stone amulet, leave her alone, she's had a long day.

Lots of mice hang out at that nasty little run-down shack called https://vanillawowdb.com/?npc=6546"">(NPC #6546) 's Farm in Dustwallow Marsh (45, 56). Killed me a critter today and got my Pest Control achievement... woot woot!