So horses don’t get hot from protein, ever. They excrete excess protein in their urine, they can, however, get hot from the sugar content in alfalfa (as not all alfalfas are made the same). Things that do make a horse hot include “immediate energy sources” like corn, oats, wheat, barley, rye, and molasses (or other sugar). Just like people, if we load up on pasta we run around like two-year-olds then crash and burn.
Some horses have food sensitivities that can make them hot like corn, oats, alfalfa, wheat, barley, soy, or grass hay.
The most common cause of a hot skinny, but otherwise healthy, horse is performance feed. You are simply supplying more readily available energy for your horse than your horse burns exercising. Try choosing a low nsc food high in slow burning/high calorie fats. The less readily available energy the horse has the less she will burn running and playing or misbehaving, meaning she won’t continue to stay skinny.
A calorie dense and nutrient dense feed may be a good option for this mare. While mine was sorting herself I had her on Progressive Grass balancer, a fat supplement, alfalfa pellets and beet pulp at every meal. I also upped her Omega 3 and Vitamin E consumption to help with overall wellness as she was on poor quality grass hay.
I would definitely consider Purina Senior Active Age as well, it is a concentrate so you aren’t having to feed large volumes to get the proper nutrition. It is also high in fat and protein and much lower in NSC. Due to the NC hay struggle I supplement a few odds and ends but otherwise the horse has finally started looking like a warmblood.