Big ^5 also from me for doing everything you're doing
I have a 26 yo Arab that I rescued over 19 years ago. He's always been a hard keep but, in the last few years, it literally takes what I feed my three Walkers to keep him at a decent weight and he's only 13.3H. Even at that his ribs show
I have read more-often-than-not about these senior Arabs suddenly becoming hard keepers and I have to wonder what's up with them in their gene pool?
So a few thoughts:
1. On quidding his hay. Has the vet looked REALLY REALLY close at his back teeth? When my 25 yo TWH started that, his teeth looked perfect --- until the vet got the little flashlight shining on his back teeth and found little hooks that were stopping the hay from being able to be swallowed.
Like your Arab, my TWH could eat anything from his feed pan perfectly but the hay was hanging up.
The vet cautiously filed those back teeth but couldn't take them way down because then there wouldn't be anything back there. Duke still quids his hay but I only find 1 - 3 "chews" every morning instead of a most of what he tried to eat.
2. The lump in the thyroid area. My first thought is also cushings. From all my readings on credible web sites and from reading what other folks post, with cushings horses, blood tests will not show cushings when it's in the early stages.
So yes, your Arab could be in the very early stages of cushings. I sure wouldn't put him on Prascend or Pergolide however. It seems like drugs have the opposite effect on a horse unless the blood results show the horse is in full blown cushings.
3. There's been colic surgery in the past and some colic issues. While my 25 yo TWH hasn't had colic surgery, he has had 4 "vet time" colics since this past March and two "twinges" that I got him out of with Banamine just this month. This horse never had a tummy twinge in the 22+ years we've been buds.
This horse has Equine Metabolic Syndrome, a/k/a Peripheral Cushings". He was diagnosed in 2007 and went from an air firm to having a hard time keeping weight on.
Point-being, my vet is suspicious he might have lipomas in his digestive tract.
At 29, your Arab might be fighting some sort of tumors in his tummy and/or digestive tract. Especially since Arabs seem really prone to tumors, regardless of color. My bay Arab has an external lump on his rump for years and when he stands a certain way, I can see a lump under the skin up in his groin area.
My first thought, however, is to have the vet check your Arab's back teeth with a microscope. My vet had checked my TWH only four months previous and gave his teeth a clean bill of health. Those hooks didn't grow on his back teeth in four short month; not when they were big enough that they couldn't be completely filed off
I am sorry I haven't given you any definitive answers; it's really tough when they hit their 20's and we exhaust ourselves and our checkbook trying to figure out what's wrong and can't
I hope this at least gives you some different ideas to talk to your vet about.
You got the Dollface to 29 and I think that's terrific - here's hoping for several more great years - even if you have to buy hay cubes and put them in a blender for him