I used beat pulp and Timothy pellets
You can give him/her some beep pulp and grass pellets and oil.
I talked to a vet a month ago about this for an OOTB that couldn't gain wait. She said to mix a little bit of canola oil & rolled oats. She said it helped 90% of her horses. Also make sure he/she doesn't have worms. That takes away weight.
Hi Abigail, you can try giving fiber to your horse in addition to his grains, you also have supplements such as Weight Builder from f
Farnam, which will help his body assimilate the food in a good way. :)
Smart pak has some weight gain supplements, but I've never used it but I know some people that do and they said it works pretty good