What purpose do dreams serve, biologically or otherwise?
Why do mammals dream? What else is there to do when you're asleep?
I know for a fact that dogs dream. I don't think they have nightmares, at least I hope not.
It’s fun to watch a dog run in their dreams, acting it out while lying on their side.
I've heard my dog whimper occasionally in her sleep. She's a rescue, so I assume that she's re-experiencing some past trauma. Whatever it might be, I go wake her up to comfort her. It doesn't happen often, but I'm pretty sure those would qualify as a nightmare or at least a bad dream (if perhaps you reserve the former for profoundly terrifying dreams).