I wonder if dogs have hypnopompic hallucinations like people do. That might explain an otherwise well behaved dog lashing out when awakened. Although it sounds like Diamond probably wasn’t “otherwise well behaved”.
Same here only it’s the crown of the head and back to the neck. They sure look like nightmares to me and I have wondered what they could be. I’m picturing being set out of a car by the side of the road and watching the car drive away.
Me too (I whisper “There’s one!”).
I’ve never seen a dog having an apparent nightmare - not that I’m disputing it happens, of course - I guess especially for dogs like rescues who have some trauma in their history. I’ve been lucky enough never to witness it.
It’s true that (at least in my experience) you never can tell if it’s a nightmare, a bad nightmare (both of which the whimpering sounds distressingly sounds like to us) or, conceivably, just plain old vocalizations out loud like when we mutter in sleep, and maybe the dog’s barking for any number of reasons in his doggie dreams and it just comes out like that.
Be that as it may, we read somewhere and for decades have always done the following, which is particularly handy if you (and dog) are in bed at a little distance from each other and you don’t need to get up:
You make a sort of loudish sound, without calling his name, like “tch-tch-tch”–like the sound of “cheese” combined with the “tsk-tsk” sound (which actually has no “k” phoneme) you make in sympathy," or the sound to alert a horse which some of you may know.
The idea, which seems agreeable (and works) to both you and the dog, is to bring him up from REM sleep, and the active moment of the dream, and he’ll stop but basically continue as asleep as he was before.
It does work. Our previous dog used to whimper loudly (“terribly”) every night, and it was distressing certainly to us.
Something I read years ago and have done ever since, if you suspect your dog is having a nightmare, hold your cupped hand to their nose… not touching, but close enough that they can get your scent. It doesn’t wake them up, but your scent is comforting enough that they stop the twitching and whimpering. It has always worked with my pack.
Not sure about dogs, but I learned the hard way not to wake my cat up when she was having a nightmare. She scratched the shit out of me. Maybe throw something from a distance.
Huh. Knew, sort of, the word hypnogogic, but not that one.
Took a while to get the pompatus of love out of my head, which come to think of it is a damn fine thing to describe your pooch.
With both dogs and child I have always just spoken softly to them. “Mommy’s here, you’re all right.” Child smiles, dog relaxes and sometimes wags just at the tip, and all is well with the world again. And all without anybody waking up and asking for walkies or midnight snacks. 
I never know if it’s an actual nightmare or just an exciting dream but my little maltese/yorkie-terrier will be sleeping and make these muted, muffled little barks. They are the most adorable thing I’ve ever heard. Unless I can’t sleep, then he gets a pillow thrown at him.
That’ll work.