The ones I remember are these...

Base game
- Razak's Opus (a Dwemer Spider) for completing the quest "The Mystery of Razak" in the Bangkorai public dungeon

Imperial City
- Necrotic Hoarvor for getting the Imperial City Challenger achievement (aka complete Speedrun, HM and No-Death in veteran White-Gold Tower and Imperial City Prison)

- Stonefire Scamp as random drop from defeating Molag Bal in the center of the Imperial City Sewers - I saw people selling this in a runebox, so you might find it at guild traders, as well

Orsinium
- Echalette for entering the city of Orsinium for the first time

Thieves Guild
- Jackal for entering Hews Bane for the first time

Vvardenfell
- Dwarven Theodolite for collecting its 7 parts (random drops from mobs) in Nchuleftingth public dungeon - the parts can be sold so you can also check guild traders

Clockwork City
- Scintillant Dovah-Fly for entering the Brass Fortress for the first time

Summerset
- Big-Eared Ginger Kitten for colleting 7 kitten care parts (random drop from mobs) in Karnwasten public dungeon - the parts can be sold so you can also check guild traders

http://en.uesp.net/wiki/Online:Pets