Horse Fable if you like horses. They have lovely art (imo), you can create cross-breeds, catch wild horses, breed for specific colors or for stats. Similar to Lorwolf, they were still developing parts of the lore/story last I checked.
Paw Borough is a cat one that's in development but soon opening beta. I could spend hours playing in their demo!
I will second Magistream as one that doesn't require as much time investment as some other pet games. I leave and come back all the time.
