My dog has had two episodes now of this. He suddenly experiences extreme weakness in both hind legs and scrambles frantically. This passes off in about 60 seconds and he then has absolutely no symptoms and looks completely normal. That it passes off so quickly and happens so rarely excludes just about every diagnosis that would account for weak back legs. Most are persistent, not happening so infrequently like this, and most do not disappear so suddenly. The first time was around July. That time, he was lying down and was unable to get up. I got him up and fully supported him until the episode passed, about 60 seconds. I took him to an orthopedic specialist who I have immense respect for, I took him immediately and the specialist couldn't find anything to account for such behavior. He said the dog had a very mild 'click' in one stifle but that was nothing to worry about, would not cause the behavior I described, and required no treatment. To be clear, this whatever-it-is appears to affect both hind legs equally. I have only observed two incidents of this, the first one in July and the 2nd today(Nov 8), and in between these two episodes the dog has had no symptoms. Absolutely no weakness, no disability, no affect at all. This episode that just happened now appeared to be worse than the previous one. I am thinking neurological, that it's some sort of spinal lesion, but how it would cause only such intermittent symptoms is a mystery. Dog has never been injured that I know of. I've had him since he was 9 weeks old. We have 'invisible fence' all around the property and an electric pasture fence on the back half of the property, but he rarely is outside unsupervised. We have plenty of raccoon and deer around, coyotes and the occasional stray dog wandering through the property. The neighbor has pea fowl. We have horses and chickens, and the back neighbor has goats and llamas. He's only 6 - a neutered male collie and other than this has never had any health issues. One thing that puzzles me is that he seems to have lost some muscle on both hind legs over a very long period of time. This is slight and even the specialist could not see it when I mentioned it. His back muscle and shoulder muscle look fine.