Maintenance for the week of December 23:
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It almost certainly is not considered a pet, but the easy way to test it for sure would just be to see if it procs any set. If it can proc sets, it isn't a pet.
I tested it to be sure, but it isn't a pet. Its damage isn't increased by the Unleashed Ritualist set, nor does it increase your Max Magicka when you wear Necropotence.