To me, it looks as though the foal has dropped low in her belly. I'm not sure it looks like a tire to me, but I can't think another way to describe it. I'm not sure that there is a problem, because she does not appear to be in any pain, just the usual discomfort of carrying around a horse in your belly : ' }
TIm ( our trainer/farrier) described it as looking like she had a "tire" in her belly. DJ is right though, she only appears uncomfortable, not in pain. What is also weird is that looking at her from her rear end, you lift up the tail and there is an obvious soft W shape that her belly shows. A big dip in the middle, not just a vein. I think this was one of the reasons our trainer thought it might be a good idea to call the vet. He didnt seem to think she was in pain , just uncomfortable, like she was about to have it soon! LOL
I can not draw a picture with words myself, so I understand the problem. As a mare gets ready to foal, the baby moves from sticking out on the sides or the bottom of the sides to a position where the belly points to the ground so to speak. The baby has turned and instead of laying comfortably (for it), it then has it's front feet and nose in the birth canal. It's rump is that bulge hanging down.