Last year I took in a 16.2 hand 29 year old emaciated mare to help a friend. Because she was very high strung and had lost most of her winter hair due to stress, she was a challenge to say the least. Within two months she looked like a new horse – still old and swayed, but new shiny hair and quality substance covering her bones. So I’ll pass along what I learned in hopes it will help you.
Transition to new feed/supplements slowly. Include the best probiotics you can afford (I used Forco). To get the vitamins and supplements in her I added them to a warm mash made from soaked alfalfa pellets with a bit of molasses and salt to help with electrolytes; I also added some of her senior feed (I used LMF Gold/Senior complete. At first I had to give this 2-3 times a day to get her back on her feet and in small amounts so it was a ‘goody’. I soaked her hay initially, we have amazing quality hay here in Central Oregon so that helped. Everything was done in small amounts because she was so distracted it would go to waste. Of course, worming and vet care are important factors, as well as just rubbing her all over each day…old ponies have a hard time scratching all those hard to reach places or getting down to roll.