Recently, Blizzard has made several hotfixes to the in-game Pet Journal to add pets that were accidentally not flagged as available. Every single pet that they've done a hotfix for does not show up in the Web API for pets.
List of the broken pets:
- Wharf Rat (speciesID: 410)
- Crimson Moth (speciesID: 421)
- Devouring Maggot (speciesID: 523)
- Silkbead Snail (speciesID: 568)
- Feverbite Hatchling (speciesID: 714)
- Emperor Crab (speciesID: 746)
- Garden Moth (speciesID: 753)
- Shrine Fly (speciesID: 754)
Example: This person (http://kr.battle.net/api/wow/character/kargath/%EC%B9%B4%EC%9D%B4%EC%A0%80%EC%86%8C%EC%9E%AC?fields=pets) has the most rare and high level battle pets of anyone else in the world. I find it absolutely impossible that he doesn't have any of the pets listed above. In reality, I'm 99% sure he has every single one listed above. He's an easy test subject for these fixes.
Here's another player with an absurd number of battle pets that has all of the above (http://us.battle.net/api/wow/character/gorgonnash/Nightdust?fields=pets).
Edit: I'm not sure if this is related but the above pets don't show up in achievement criteria correctly either (though the achievements are still complete-able).