I'm not talking about pet jobs with pets capable of tanking.
Plenty of mmo's have pet jobs as dps, and healer/buffer jobs with pets aren't uncommon either, but I've never seen a tank with a pet.
Conceptually, it could be neat. Have two pets, one defensive--that provides auras or buffs or debuffs, one offensive--that just does damage, but it would probably be hard to balance.
Warcraft: I do know about DK's Army of the Dead but that was just a cooldown and the pets had no intelligence. I know about Unholy's minion but unholy was not a tanking tree.
Spec'ing properly is a gateway drug.
12 Million People have been meter spammed in heroics.