Over at press 5 to capture, Ratshag has come up with five new ideas for pets from Burning Crusade content, to complement the ones showing up in Vanilla or formerly-in-Vanilla (Naxxramas) raids. There's some great ideas in there - I'd definitely go farm for a Lil' Reliquary - But why stop at 5?
Lil' Curator: Magical pet. Drops in Karazhan. Spawns its own helpers. Occasionally stops and regenerates mana despite the fact that magical pets don't even USE mana. Drops pet tier for everyone but you.
Arakkoa Hatchling: Flying pet that doesn't actually fly, but uses a lot of magic, humanoid, and undead attacks. Capturable in Skettis. Level 20 ability is a hefty self-heal called Dark Crystal.
Mini'ru: Undead pet with magical attacks. Patched into the game after Blizzard realizes the majority of the other pets have been caught. Drops in Sunwell. When he dies, he turns into a different pet with an entirely different set of abilities for one round. Looks frickin' awesome either way.
Loot Reaver: Mechanical. Drops in The Eye. Not actually very hard. Everyone has one after a month but no one uses it.
Baby Kraken: Aquatic pet. Found by fishing in Serpentshrine Caverns. Has a unique ability that does aquatic damage and forces the current front line pet out of the battle (and probably into a pit of pirahnas, knowing my luck).