My dog, The Dude, suddenly had a pop eye. I noticed it a week ago today. The white, third eyelid, was raised halfway up his left eye. By the next day the lower lid looked like it was pulled away from his face. By the third day - when I called the veterinarian - I couldn't even look at that eye without feeling ill myself. It didn't seem to be sitting in the socket correctly.
I had checked the Internet and diagnosed a prolapsed third eyelid. He certainly looked like the pictures I found out there on Google. Fortunately, I am not a veterinarian, as my dog's doctor thought it might be Horner's Syndrome instead. He took some x-rays and gave me some eye drops. We're due to return next Monday for a follow-up visit.
He's already looking better. I think the veterinarian's diagnoses was better than mine, fortunately.