My horse was about 100lbs under weight when I bought her last Oct. at the time she was eating half a bucket of sweet feed "whenever they rode her" (which I am guessing was NOT often) and whatever grass/weeds she could find in a barren pasture. Winter coat was very dull and basically non-existent.
Consulted with the vet and her diet was 4 flakes of grass hay (2 each AM & PM), rice bran in the AM and Senior Feed AM & PM. Vet said no alfalfa, she would most likely founder going from an inadequate diet to getting alfalfa (we asked since we have alfalfa for another horse).
Within a month, she had a good, shiny winter coat and by January I think? She was a decent weight. She's now slightly chubby and we're working on converting that to muscle, she's still getting Senior Feed since she's getting worked pretty hard. I ride her 2x per week and my Sister-in-law either rides or lunges her 3-4x per week. We can't work her for very long (she tires easily due to previous poor nutrition and no exercise) so we go for short sessions as often as we can (or the dang snow allows).
She's looking great, gone from slow, aimless wandering around the pasture to racing my SIL's TB down the fence line turnout and shocked us all by having the energy to go bucking across the arena when asked for a canter this week.