The worm is your exotic Tenacity option if you need a really hard-core single-target tanking pet. It doesn't have the gorilladin's ability to tank multiple targets at the same time, but you can't do much better than Acid Spit for single target debuffing over time. The more physical damage-dealers you have attacking that target over the longest period of time possible, the more you'll see its effect stack up. It looks to me like the best boss-tanking pet you can find in a party made up of mostly physical damage-dealers rather than magical, if you can get your friends to let it tank the boss, of course. In PvP, the effect doesn't wear off as fast as the devilsaur's ability, so you might find it somewhat useful there as well, if you need a pet with more survivability.
BRK showed us worm a while back, and of course Wowhead can show you all its 6 colors (as well as the upcoming Northrend version). In Azeroth, the most likely place to find these is in Silithus, or if you're in the Horde, you can just hop down to Ragefire Chasm and tame one there.
If you need a non-exotic pet with armor reduction, you can try out the wasp and its Sting, though that's a Ferocity pet for damage more than tanking. If you want a good Tenacity pet for PvP before Northrend, then the crab and its Pin ability might be what you're looking for. In Northrend, however, you're going to want a Rhino for all your PvP Tenacity needs, without a doubt.
Patch 3.0.2 guide to Exotic Pets: Worm
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