I feel Marty is right on with the KISS...Severely emaciated horses need to come back slow, hay being the biggest part of their diet, and just plain old grass.Something to consider is that most vets treat the little guys same as full sized horses. Big problem here is,their intestines cannot handle tough course hay like the full sized horses can, alfalfa is very rich and can be very course. There is something else which comes out of left field, it happened to a mare of mine, only reason I will mention it.
This mare went to be bred, stayed a month, came home thinner and with screaming squirts, equine vet I worked for came out ran normal tests along with fecal, no answers. 3 weeks later my boss was out of town, had a fever on another horse had to call out another vet...did farm calls emergency only he was primarily small animal doc. Treated the geldings fever so what the heck asked him to look at the mare. He did another fecal no other tests, his tech called a couple hrs later, telling me to get to the office immediately to pick up antibiotics. They found coccidia when they ran the fecal. This is very unusual to see in a horse, my boss said she never even looked for that. Ran the antibiotics and she cleared up. Bad part about coccidiosis? (not sure if thats the correct name) is stress can cause the diarrhea to return. Just another thing to consider.If you opt to run a fecal, you would need to discuss this with them since it is rarely seen and not looked for or considered.