There are several weapons that scale with your pet, Replica Minotaur Knocker for example goes from Tier 1 to Tier 11 as your pet goes through those tiers. However, if you're looking at the weapons in the armoury it along with the others are shown as their minimum tiers.

Would it be possible to be able to see what tier they'd be for the chosen pet, say, in the Customize option in the Choose Weapons window? Or maybe an option to make this available in the Armory tab?

For the longest time I didn't know there were weapons that did this so I didn't use them because why would I equip a T1 weapon to a T7 pet? Even now I often forget which weapons do this and have to go look them up on Subetalodge.