My 3yr filly, Rem, has always had a small appeitite. Eating only enough to substain a somewhat normal weight. She always has hay in front of her (Really good quality, nutrient tested, alfalfa/timothy mix) and since being in training she also gets a small amount of complete feed, whole flax seed and powdered vits and minerals every night. She hardly cleans up her hay and the barn owner said she actually eats less than half of what the other coming 3yr olds do.
I've noticed that with the warmer weather she's been eating less and less. She is otherwise a very healthy horse, with a good amount of energy and good work ethic. She is happy and waits and the gate for you and has bright shiny eyes. The vet saw her last month and did all her dental work, so her teeth are not the problem.
I've attached a picture of her that I took today. She isn't really skinny but can't afford to lose anymore and we are desperatly trying to get more weight on her (We like our horses to be borderline chubby )