I didn't independently verify all 400+ pets yet, but comparing Mephisto's and Guenivere's description to what they actually do:
* happiness bonus is identical
* arcane ability scales and the description is somewhat vague, but it doesn't include 50% damage reduction nor 35% damage since those aren't on the pet
Not sure where you're getting your info from here. Can you be more specific to what you're actually seeing vs. a rumor?
-ALS
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