I looked up the Hydrus Clamshell (Pearl Included) on SubetaHQ and it says that it's a minion and a weapon, but I can't seem to give it to my pet so is it actually a minion or just a weapon?

I have the option to equip it to my pet and to add it to his treasure but not give it to him.

Subeta HQ isn't always right. They have missing information, and they occasionally make a mistake. Or an item was intended to be, or do, something and the people behind Subeta changed their mind. Or it could be an error on the part of Subeta - if an item was redrawn or there were database problems, the original parameters of the item might not have been restored fully. You could always submit a ticket asking if its supposed to be a minion, or if it was an error on the part of Subeta HQ.