Cory Stockton, lead content designer for World of Warcraft, has described the origins of pets in the game, be it a combative pet or just a rare vanity pet you can tame for bragging purposes.
Most are just modified versions of the ones already in the game, some are re-skinned. Stockton used the Cataclysm PvP raid zone Tol Barad as an example of how ideas for pets are come up with.
"We requested a seagull from the character art team and my seagull came in," he said regarding the area which has a group of Tauren fishing. The bird is currently faction reward for boosting rep in the area.
"I got some basic animations on it. Like it being perched, it flying around in circles. I thought the seagull looked really great, so I wanted to make a pet out of it. At that point I just requested a couple extra fun animations, so we have an animation of the seagull flying in a loop and we have an animation of it pecking around in the ground...it ended up looking so cool, once we got it from the character team.
"A lot of times, an asset will come in and we won't even know that we're going to make a pet out of it. But when it gets in a designers hands, things like that can end up becoming much more than we originally envisioned."
However, there are limitations as to what kind of pets can be added, and according to Stockton, if options were limitless, players would see a lot more critters running around than at present.
"We want the pets to feel like they at least came from the universe," he said. "Even the pets you see that are sold on the Blizzard pet store, they come from the Blizzard universe, so they make sense. We definitely don't want to put something in the game that's so out of context that when you see it, it pulls you out of the world that you're in."
Stockton goes on to say that more pets can be expected in the future, as WoW has "a long time to go".
There's an awesome pet guide posted on PlayingoftheGods, which describes how you can attain each pet in the game, thus far.