I get REALLY riled up about horse weight. There is a *HUGE* difference between the right weight and too skinny, and there is a really easy way to tell if your horse is the right weight, too fat, or too skinny and it has nothing to do with how much rib you can see.
On a fit horse, you should be able to just barely see the rib cage when they flex, and it's not a death sentence if you can see the shadow of them when they're standing there.
But the most important and reliable area to detect the health of a horse's weight, in my opinion, is over the hips, where the flank and the spine connect. On a too skinny horse, this area will be pointy, as the hip-bones will jut out. The skinnier the horse, the pointier the bones. On a fit horse, this area will be rounded or nearly flat depending on age and condition of the back, but either way you won't see pointy angled bones jutting out. And on a too fat horse, this area will appear swollen with the spine sunken in between the hips.
I cannot stand people that get all upset because a horse's ribs can be seen. Not glaringly sticking out, mind you, just seen. Like if they can detect that the horse has ribs, period, they are too skinny. Not the case. And weight is super important, especially for pastured horses, in preventing founder.
There was a neighbor near my old ranch that used to call the humane society on us every two months or so because some horses had ribs showing, which led the barn owner to overfeed them to stop that lady from calling her in, which led to colic and other nasty things.
Ugh, this really gets me all angry, haha.... I just don't like people accusing me of not taking care of my animals.