That's a Lot of Pet Food

These are the pets that are going to give you some problems trying to collect:

*KALIMDOR*

Winterspring - Snowy Owl: Only available in the winter
Felwood - Minfernal: Not 100% sure how to get but apparently you have to kill the infernals in felwood to get one to spawn. I was killing them for ~30 minutes without luck before moving on. He is not connected to the rare spawn. I killed the rare spawn without him popping.
Dustwallow Marsh - Spawn of Onyxia: Only 1 is up at a time and appears to have a 5-10 min respawn. I was not able to find any attached to other mobs around Onyxia's layer.
Northern Barrens: Cheetah cub. Only has few spawns. Look under the large trees where the cheetahs bask in the shade.
Southern Barrens: Giraffle Cub: Not hard but like the cheetah only limited spawn points.
Silithus - Qiraji Guardling: Only spawns during the summer
Silithus - Scarab Hatchling: Very limited spawn points. I got mine by battling a scorpid near the wall who had one of these guys attached. I suggest farming mobs near the wall and hoping you see one attached as well.
Un'Guro Crater- Diemtrodon Hatchling. Only spawns southwest part of the zone with Elder Diematradons. If you don't see any its been suggested to kill the Elders to get a hatchling to spawn. I tried for ~30 minutes without luck and moved on.
Tanaris - Sea Gull: Theres a ton of them but they're flying way out of range to battle. Don't go after them directly but battle mobs near the water and youll see them attached in battle.
Tanaris - Silithid Hatchling: Only spawns when theres a sandstorm and, obviously, only in the silithid camps.

Eastern Kingdoms

Duskwood - Widow Spiderling: Only comes out at night.
Cape of STV - Baby Ape: Only spawns when its raining.

You must acquire 400 Unique pets in order to earn this, Multiple copies of the same pet do not grant credit towards this achievement.

If you've been having some trouble finding the pets and you've been using the Safari achievements to keep track of what you're missing, don't forget that there is no Safari achievement for Deepholm.

At the very least that's the Crimson Geode, Crimson Shale Hatchling, Crystal Beetle, Deepholm Cockroach, Fungal Moth, and Stowaway Rat -- six pets I know of available but not currently listed under any Safari achievement.

The Stowaway Rat is found in the shipwreck southwest of the central hub, and everything else I listed is found in the Crimson Expanse on the east side.