Every time I go to town, I pass by a horse in a pasture. A pasture is that the grass is up to his knees and he always has his head bogged right down in the grass, not coming up for air a bit.
However, he's a little on the thin side and I don't quite get that. The first thing I think of is a good worming but he seems to be pretty slick and not pot-bellied a bit. Still a good worming wouldn't hurt.
Since I see him in this little pasture every time, I wonder if the owners ever take him in for proper feeding. He may be a geezer gelding for all I know but, in my experience, that's not a reason for him to be thin. He seems to be, maybe, a TB or TB cross but that's still not a reason. I don't think he's being worked a bit, just holding the pasture down and keeping it mowed.
So, what's the deal on him then? I would say do a blood panel and find out if everything is normal. Take him in for breakfast and supper and put oats in front of him along with alfalfa and a good supplement for weight building. Check his teeth but don't get carried away (more on this later). Take all stress off of him. After doing these simple things, he should start gaining weight where you notice it in less than a week-ish.
I can't tell the number of horses I've seen over the years where someone said their horse wouldn't gain weight, only to sell the horse to someone else who plumps that sucker right up. It's a matter of finding the right combo and then DOING it, following through. Not only saying that 'he's been like that for years and won't gain weight'. Not true. If some combo of feed doesn't work, then go to the next thing but something will work. Only if he's so debilitated by something, then he won't but it'd have to be pretty big to keep him from gaining.
This has been bugging me for a while and now it's off my chest... :-) Now, go ride your horse!