I have an 9 yr old Arab mare with a fairly long back and no condition along her topline (never foaled). She has never been put under saddle and is fairly lazy, so NO muscle. She stands by the round bale all day and night, like all the horses, cows and goats, etc. She gets whole oats twice a day in small quantity in the winter.
She is really the only one that has a hay belly. One rescue had a faint, faint one and could slightly feel ribs along the topline; however, he was low in the pecking order, and that is the reason for the the slightly lower weight. We had 7, counting our rescues until 2 were adopted on Tuesday, and everyone lookes great except this mare. Her belly is really big and hangs low. Remember though, her topline is a mess . . . No condition at all and her back is overly long, but you can feel ribs. . . She was in gorgeous condition over the summer/spring/fall.
Am I looking at a nutrition problem? I will say, I know the hay isn't great quality, BUT it is fairy good as the other horses (even those on no concentrate), the dairy heifer, the dairy goats and donkeys all look good. This mare does need her teeth done if I can ever get our equine dentist to call me back and get out here. . .

The photo doesn't show how prominent her spin is. . .and though the ribs are faint, they are easily felt around the top line. . .not so much further down. The belly sticks out pretty far on each side, as well.
The chance of her being wormy is fairly slim. Wormed in fall, wormed 2 weeks ago with Quest, as a precaution. The belly changed not at all from that.
I'm going to take her off 24/7 hay and start a complete feed with hay supplemented and see how that does. . .
Just curious about opinions.