Yes. Possible brain tumor. But not necessarily.
My Billy has a good old run when asleep. Not as drastic as that but from his leg movements and the yelping sound he makes you know he is chasing rabbits because he only ever makes that sound when chasing rabbits for real and even then only when he is running just inches behind one. (He knows I don't like it if he catches one).
He often does this before or after his 10:30pm walk which is when he goes daft running through the Hospital grounds looking for and chasing rabbits. So he is probably looking forward to doing that when he falls asleep while waiting for me.
And there are still some "experts" who will tell you dogs don't really dream.
