I've been told that my horse is too fat by an owner of an overweight trotter. She failed to notice, though, that my gelding is a light draft and is not supposed to be as narrow and lean as her horse should be, so I was given advice on how I should cut on his feed.
Also, when I just bought my gelding and was a first-time owner, the BM of the barn where he lived helpfully gave me a tip never to spend more than three hours a day with him. Why? Well, apparently horses become too attached to humans if so much time is spent with them and become "wild, dangerous and uncontrollable", if so.
I was also "taught" that horses can be given only very small amounts of carrots or a rare apple, but no other vegetables or fruits at all, as any of them bloat horses up and induce colic.