They're those cute and adorable creatures that follow your World of Warcraft character wherever they go. There are achievements that require you to collect a lot of these little guys, 50 and 75 of them. Not to mention more new pets that came with the 3.2 patch. Raptor Hatchlings!
Here Yadiera has gathered an almost complete list of all the World of Warcraft pets that are available in game for Alliance or Horde. Pets obtained through a Quest or with high Reputation. Even all the pets dropped by specific mobs or bosses and seasonal pets. Listing costs, drop rate, who to get it from and how you get it.

There are over 100 vanity pets available in World of Warcraft, without the special promotional pets, or trading card pets. Collecting 50 - 75 of these pets can be a challenge as most of these pets are rare drops from specific mobs throughout Azeroth, only available to get during one of the WoW Seasons, or requires high reputation with a specific faction.

Full Guide: Here