A lot of people get these OTTB's not realizing that their nutritional needs are a lot different than your stock bred horses and easier keepers. And get way in over their heads, and this poor guy suffered for his owner's lack of knowledge
Agree that people go in ignorant, horses suffer for it, but I don't agree they have different nutritional needs, and they can indeed just as easily be 'easy keepers' when fed/managed right. It's the way they are fed & managed as racehorses that tends to cause gut probs, turning them into 'hard keepers'. It is often more about good, 'low GI' feed, free choice hay/grazing & treating the gut issues effectively more than them needing more calories.
I had 2 horses looking on the thin side like that(not that thin but close) at the end of winter this year(that's a first!). One I wasn't worried about - he's usually fat as mud & he was getting enough - albeit just poor - grazing/hay. & they all get regular nutritional supps. He is now fat as mud again, when the grass picked up. The other didn't get as thin as him, but she is an OT horse who's ulcery & so needs TLC. She, like my other horses, actually did fine on the poor grazing & hay... but then dropped off quickly when I ran out of the 'Gastro Aid' & couldn't get any more for a while, that seems to be effectively managing her ulcers(but obviously not getting rid of them completely). As soon as I got more & she'd been getting it for a week or so, she picked up again.
I agisted a while back, at a paddock always full of (too)rich grass. All horses were fat as mud, including a couple of TB's. Then a new girl got an OTTB that was skinny, and being TB, she told us he was innately a 'hard keeper' & so needed large, rich feeds. She expressed her surprise and confusion that there were other fat TB's there that weren't hard fed. She'd come down with a huge bucket for him, once or twice a day, full of grain. She also regularly chopped & changed feeds, because she wasn't seeing him gain any weight after a few weeks, so would declare that feed didn't work & change. He kept losing weight & then one day got a phone call from the property manager that we would need to remove our horses immediately. The TB had died of colic and the girl had told the manager she thought it was due to... botulism or some such, from ducks being in the paddock(I remember asking here at the time about the risk of botulism from ducks...)! So they declared the paddock unsafe for any horse... Unfortunately the girl cried on social media about losing her poor rescue horse & someone gave her another OTTB... which I heard thru the grapevine mysteriously died of colic too.