Familiars for mages, permanent pet companions for druids ... or rangers. (Not summons that vanish when you leave an area. Critters you can NAME, level up, pick talents for, maybe even augment, and stick around, like hunter pets in WoW, or ... loveable Mabari.)
As with most games, the familiars are not battle pets, but might provide the mage with some magic bonus as long as they're around ... and can provide advice/interact. Black cats, imps, etc.
The ranger/druid pets ... well personally I don't think spiders or bears make good constant pets, but maybe wolves, and panthers, yah. Give your animal companion some stealth, and let him go out and scout for you ...
Personally, one of the things I like about WoW are the hunter pets, and their variety of abilities (although in essence other than that, and the three "families" of attack, defense, and middlerange pets, they're pretty interchangeable).
I'm in favor of a semi-permanent fifth "pet slot" along with the four slots for PC-NPCs. Pets and "NPC temps" could go there.