At 26 years of age, hay may not be providing much in the way of nutrition for a horse due to the wear and tear on teeth. Having his teeth checked is definitely recommended as there may be issues which lead to uneven wear and affect chewing even more than normal in these old guys. But still with other changes in the digestive tract that occur with age hay becomes less and less digestible with age. That being said, if the horse is still eating hay well (not quidding and not passing it out in feces basically unchanged) then the first place to start on a diet plan for weight gain is adequate forage. For most horses you want to feed 1.5% of their body weight in forage a day---for 1000 lbs of horse that is 15 lbs of forage. For a horse that you want to increase the weight on, you want to increase the amount of forage. Free choice forage is the best way to go, but if you can't do that then increasing to 2-3% of the horse's body weight is good.
Besides hay, if the horse is on a feed designed for senior horses she needs to be sure that she is feeding it at least at the amounts recommended on the label and even above that amount for weight gain.
Fat in the form of oil is a great choice for these senior horses because it is a very concentrated source of digestible energy and easily digested. As Luvs2ride said, 1-2 cups a day can be fed for increased energy and body weight. Any vegetable oil can be used but for senior horses I would stay away from strictly corn oil as it has a high amount of omega 3 fatty acids which feed into the inflammatory cascade and thus exacerbate inflammatory issues like arthritis.
Licensed Veterinary Technician