In Greyhoundland, we call it "chasing the bunny" It's actually quite cute, and I don't think it comes from abuse. Many dogs dream and greys, especially, have a funny way of doing it. They'll twitch their legs, thump their tails, snarl their teeth, and, like a poster mentioned before, they'll even wake themselves up with their own whimper. It gives a gray area between animals and cognitive thinking, but it does make me wonder what they're doing in their sleep!