Just wondering, I know that you can with Magical Keepsakes box, and with the V2 Zapper. But there are any other ways? Thanks in advance! :)

there's an item in the cash shop that does, i don't remember how many slots and random events.

Thank you!

I believe there's an option with the loyalty boxes, that you get every year after two years as well. If you choose that, all your pets get more slots, though I couldn't tell you how many.

The Cash Shop Item is called Multi-Compartment Keepsake Box and it gives your Pet 10-15 additional treasure spots. C:

The Pet TC slots that the Loyalty Box says in the description;

Thanks everyone for your help!

You can generate extra spaces at random by just doing something on site that is refresh-intensive like Leviathan Harpooning or Mystic Spindles. This is because there is a random event that increases a pet's Treasure Chest.