A thin horse tempts owners to simply pour in more feed, but condition goes on safely only when you know why it came off. Feeding more of the wrong thing, or feeding plenty into a horse whose teeth, gut or herd position are working against him, adds cost and digestive risk without adding cover over his ribs.
Find the reason before you fill the bucket
Have your vet check a horse who has lost weight without an obvious explanation, and ask about teeth and worming at the same visit. Sharp or missing teeth make chewing hay painful and inefficient, and you may see the evidence as quidding, balls of chewed forage dropped around the stable or field. Check the social picture too: a horse turned out with stronger companions may be walked away from every hay pile, and no ration helps a horse who never gets to stand and eat it. Age, pain and recent illness all change what a horse can use from his feed.
Build the ration on forage first
Good hay or haylage, offered almost constantly, is the safest foundation, because the hindgut is built to work on fibre and a steady trickle of forage keeps it healthy while weight returns. Lucerne-type forage, where it suits the horse, adds protein and calories within that fibre base. Once forage is maximised, add digestible fibre feeds in the bucket; soaked feeds such as beet pulp are gentle and palatable, and the day's bucket feed should be split into two or three smaller meals rather than one large one. Increase any new element gradually over a week or more, and be cautious with large starchy meals, which a thin horse's digestion handles poorly and which can do harm before they do good.
Stop the calories leaking away
A horse who is cold burns feed standing still. Rug a thin horse appropriately for the weather, check the rug fits and is not rubbing an already bony frame, and make sure shelter is somewhere he will actually use. Ease the workload while condition returns, because asking a horse to work hard and gain weight at the same time splits his feed between two jobs. Keep water and salt available constantly; a horse who drinks poorly eats poorly too.
Expect weeks, and measure them honestly
Healthy gain is slow: weigh-tape and condition-score every couple of weeks, and take photographs from the same angle in the same light so you can see the topline and ribs changing. Record his feeds, scores and any changes as they happen, because 'he looks better' is unreliable after a month of daily looking. If a sensibly built ration produces no gain over several weeks, go back to your vet rather than simply feeding more, since an underlying problem is the likeliest explanation. Keeping the weight records and ration changes in your KinsLe Pet Passport means your vet sees the whole picture at a glance on the next visit.