Some of our success stories are also a warning for others.
We have over the years handled horses that were on their last leg, literally, that if they fell, they may not have the strenght to get up again.
I would say more than half of those were horses with terrible mouth issues, that just could not eat enough and were starving to death.
It takes more than a few bites of food to keep a horse alive and a few bites here and there is all those horses could eat thru the mouth pain, so they were starving.
So, I say, if a horse is really, really skinny and others are fine, there is not just pure lack of food there, please first see to their teeth, before you do anything.
More food just won't work, get a vet to look at the horse and get those teeth tended to, as that probably is part if not all that may be wrong with a very skinny horse.