Northrend Safari

I gathered all pets required for this achievement and sorted them by location:

Borean Tundra (20-22)
Borean Marmot (critter)
Shore Crab (aquatic)
Oily Slimeling (magic)

Dragonblight (22-23)
Arctic Hare (critter)
Dragonbone Hatchling (flying)
Strand Crab (aquatic)
Tundra Penguin (aquatic)

Grizzly Hills (21-22)
Fawn (critter)
Grizzly Squirrel (critter)
Imperial Eagle Chick (flying)
Mountain Skunk (critter)
Mouse (critter)

Howling Fjord (20-22)
Chicken (flying)
Devouring Maggot (beast)
Fjord Rat (critter)
Fjord Worg Pup (beast)
Maggot (beast)
Rabbit (critter)
Rat (critter)
Roach (critter)
Skunk (critter)
Snake (beast)
Spider (beast)
Squirrel (critter)
Toad (aquatic)
Turkey (flying)

Zul'Drak (22-23)
Huge Toad (aquatic)
Water Waveling (elemental)

Sholazar Basin (21-22)
Biletoad (aquatic)
Stunted Shardhorn (beast)

The Storm Peaks (22-23)
Arctic Fox Kit (beast)

Icecrown (22-23)
Cockroach (critter)
Scourged Whelpling (undead)

Many of them can be captured in few locations.

I hope that helps. Happy hunting.

NORTHREND
Here is a list of all pets that can found by zone. I've done a fair amount of research and through personal experience, this is what I've come up with. Not everything is perfect, and mistakes are made. I have linked some wonderful addons/info for those who are hardcore pet owners. If you do find an error, please kindly leave a note with detail, so I may edit. The numbers next to the zones are the levels I've encountered. However, if you look at the ingame map, you'll see the levels of the pets in that zone.

Happy Hunting!

If you're like me, and you enjoy rare pets, I recommend this addon: Pet Quality Notifier

Links to additional Safaris:
Eastern Kingdoms Safari
Kalimdor Safari
Outland Safari
Pandaria Safari

Wowhead Pet Guide

PATCH 5.1 BOSSES: Only on this continent
Naxxramas
Fungal Abomination - Loatheb
Giant Bone Spider - Maexxna
Stitched Pup - Gluth

NORTHREND
Unborn Val'kyr - hard little sucker to find, can be in any of the Northrend zones
Borean Tundra - (20-21)
Arctic Hare
Borean Marmot
Nexus Whelpling - Coldarra - (Patch 5.1)
Oily Slimeling
Shore Crab
Tundra Penguin
Crystalsong Forest
Mouse
Squirrel
Unborn Val'kyr - This is actually all over Northrend, however, people have told me their best luck is here and Icecrown.
Dalaran
Rabbit
Squirrel
Dragonblight - (22)
Arctic Hare
Cockroach
Dragonbone Hatchling
Shore Crab
Stand Crab
Grizzly Hills - (21)
Fawn
Grizzly Squirrel
Imperial Eagle Chick - annoying to find. Did find some near Camp Oneqwah, way out east.
Mountain Skunk
Mouse
Howling Fjord - (20-21)
Chicken
Devouring Maggot
Fjord Rat
Fjord Worg Pup
Grizzly Squirrel
Maggot
Mountain Skunk
Rabbit
Rat
Skunk
Snake
Turkey
Toad
Icecrown
Cockroach
Scourged Whelpling - best caught at a server restart
Sholazar Basin - (21)
Biletoad
Emerald Proto-Whelp - (Patch 5.1)
Snake
Squirrel
Stunted Shardhorn
Storm Peaks
Arctic Fox Kit - ONLY when it is snowing
Arctic Hare
Mountain Skunk
Yes, these two do exist in the zone, though not as much as you would like to see them.
Wintergrasp
Rabid Nut Varmint 5000
Zul’Drak - (22)
Arctic Hare
Huge Toad
Snake
Spider
Water Waveling

Easiest way to get this:

Open your pet journal and set the filter to 'not collected' only.
Type in the zone name.
Click a pet and mouseover the pet icon (the one in above the 3D model).
If it is unique to the zone hunt it down, collect any of the others that you come across.
Move to the next zone and repeat.

Once you have all unique zone pets you should have most of the others, too.