Just like Outland, there are many pets in Northrend, most of them are similar to pets you have seen before, but with unique skins and occasionally a different ability. Then there are the dragon pets, which are more numerous than on other continents, but often with difficult spawns. Blizzard is still working on adding new pets to Northrend, including the Unborn Val’kyr, which is likely to be tough to catch as well.
Rather than go zone-by-zone, I’m listing the must have and hard to catch pets and you can look up other pets on WowHead. Also, don’t forget about the Argent Tournament which has a number of cool pets, Breanni’s shop at Dalaran, which has a number of nice pets, and the sewer market in Dalaran, which has the scary, but fun, Ghostly Skull.
The Required List
Nexus Whelpling. The consensus is a slow but steady spawn rate. The abilities are the same as the Azure Whelpling. Not an absolute must, but very cool and tamable pet.
Emerald Proto-Whelp. According to the WowHead comments, there seems to be some voodoo involved in getting these to spawn. I capped mine by persistently checking the location while looking for the Scourged Whelpling and waiting for snow in Storm Peaks.
Its abilities are similar to the Emerald Whelpling. The combination of the strong self-heal, the magic attack, and the shield ability make this very good against the later tamers you will come across. Catch one, upgrade ASAP, and level it. You will use it.
Water Waveling. This pet is very good at killing other Elementals and does well against the few Mechanicals you come across. It spawns quickly and you have a good chance at getting a rare. In fact, you will likely come across more than one, so try and go for the stamina/speed breed over the more balanced breed as hitting first is important with this pet. Catch a Rare, level it, you will use it.
The Hard to Catch List
Oily Slimling. Slow spawning and hard to catch in a rare. If you already have a good ooze, you don’t need it, but it’s still a good pet.
Scourged Whelpling. Very long spawn rate that may have been improved in the last patch. They are perfect for killing Dragonkin, but you will likely have to take whatever you can catch and use an upgrade stone unless you are very lucky or very patient.
And that’s it. There are lots of other cool pets, but if your goal is to get to Pandaria, you can come back for them later!