From my brief playing around (only just finished Divine Divinity, so haven't started OS yet!), I've made the following observations:
* The pet summon ability only exists on the toolbar - if it's removed, I have no idea how to get it back
* Building off the above point - removing it from one character's toolbar essentially makes the pet unique
* Summoning another animal (e.g. giant spider) will dispel your pet
I would also like to hear from others!