I've recently received this pet had issues to use it. Double-clicking doesn't work, standard pressing 'E' to use it didn't work. Could only use it via R-click and select Use.

Now I also see that this pet uses additional inventory slot.
If you are more loyal over the years, and you'll always get new pets which use one slot you'll don't have enough slots for other items then...

Why aren't items as pets, disguises, .. stacked?
You have a max of 110 slots, ..and sometimes this isn't even enough