This hippopotamus (sorry) was once a fairly fit, if stout, riding horse:
Look! He had a neck with an actual throatlatch then!
He was so obese, the owner didn't dare ride him in his condition, and was afraid founder was imminent.
She boarded him at a private barn with strict diet instructions, trying to hand exercise him as much as possible.
Despite the owners' efforts, the weight was slow to come off. As spring approached, he'd lost enough to be able to cinch up a saddle, and exercise increased.
April 2007
However, the barn owner reportedly took it upon herself to feed him when he looked "bored" or "hungry" which slowed the rate at which he was able to shed the extra weight.
By the end of summer 2007, he was moved to a different barn, where his diet was better regulated.
It took over a year to get him back to an acceptable weight.
Here he is now, still a big stocky boy, but no longer dangerously obese:
Obesity in horses is a dangerous condition. It can cause irreversible damage, and should be considered as undesirable as malnutrition.
IMO and that of the owner of this horse, it is easier to add weight to a horse that needs it, than to remove excess weight from an overly fat horse.
There will always be a horse that stays thin no matter what or how much you feed it ... there will always be a horse that stays fat no matter how well you regulate its diet. I'm not on the warpath against people who happen to own such individuals and do their best to manage their weight and maintain a healthy condition. However, if you're overfeeding your horse because you LIKE to see them FAT ... STOP!!
Think about what is best for the individual horse.
Then feed accordingly.
Then feed accordingly.