I am NOT positive, but...
I think that Stalwart is a pet that can get to level 50, and is slightly stronger for the Horror. It has a shield, so I'm guessing it has more armor. I noticed around level I'd imagine that, like a Beastmaster, the basic pet (horror) is capped at 30. Edit: Stalwart is also capped at 30. The difference is that the Stalwart is a tanking pet, which the Horror is just extra damage.
As for the others, they can go up to level 50, and I believe they have upgraded abilities and stats compared to the other pets.
Zealot: melee DPS pet (think rogue)
Knight: melee tank pet (think warrior)
Revenant: magic DPS pet (think mage)
I personally run with a Knight while soloing and a Revenant in Warfronts. The Knight pet gets buffs that decrease incoming damage. The Revenant does good burst damage, while I believe the Zealot does small amount of quick damage. I like the Revenant because it stays back with me, out of the thick of things, and thus lives a bit longer. I'll try and test it out and get some specifics soon.
Did some more digging. Note that the horror is a rogue-type, and the stalwart is a warrior type. The reason the horror started to lose aggro is because one of its two abilities lowers aggro when it deals damage.
Edited, Feb 8th 2011 3:29pm by LockeColeMA
Zealot: Jagged Wound, Soul Rend, and Blood Spike (I think)
Knight: Bone Shatter, Bone Strike, Grave Touch
Revenant: Ruination and Oblivion (not sure of others).
Note that I think Stalwart shares two of the Knight abilities and Horror shares two of the Zealot abilities.
Edited, Feb 8th 2011 3:48pm by LockeColeMA
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