To equip your pet, click on Storage on the Town screen. At the bottom of the screen, click on the Hero you wish to equip the pet to (in this example, we use the Sorcerer).
Click on the Paw icon at the top right of your screen.
All currently unequipped pets will be displayed. Select the pet you wish to equip by clicking on it.
Drag the pet’s icon to the Equipped Pets slot on the lower left.
You can unequip or swap your pet out at any time by returning to Storage on the Town screen!