As of patch 3.0, this bug has been removed from the game.

Caster stats was a term for tameable beasts which had higher than normal intellect and spirit when tamed, but lower than normal health, attack power, and armor. At earlier levels, the difference between a caster pet and a non-caster pet were not as pronounced as they were at high levels. As a result, the pet's attacks would become progressively less effective and its overall survivability progressively lower in relation to level; therefore, beasts with caster stats were generally avoided for taming.

Former caster pets:

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