Unfortunately, the pic didn't work....
15 is not terribly old for a Horse...The Great Idle Dice won the Lake Placid Grand Prix at 23....so Did Johnnys' Pocket...and Balbuco won Ox Ridge at 21....
For the Moment was still winning at 20...I could go on & on...especially the Equitation Horses....

Have the Vet do a Complete Blood work up when they come out...fix their feed accordingly...Vitamins also...do they need B-Complex??? ESE????selenium is great for older horses....
Has an Equine Dentist Floated their teeth professionally????
Have they been wormed professionally???(Not The Paste Garbage)
There are Lots of feeds that just bloat (Bran,for example) they put weight on...But is Hollow weight...they need Nutrition...not Just Bulk...Some people use alot of Beet Pulp for weight gain...It is good...and you can mix other good nutrients with it..But again,It is used primarily as Bulk...not nutrition...so do get your vets advise and Be Careful...Go Slow...You don't want to founder the poor pony,if it has not had proper nutrition....get a Detailed feed Menu from your Vet and Stick To It...if no results...get vet back to Re-Evaluate..

You are doing the right thing getting the vet out...Just make sure that you do One Stop Shopping while Vet is there....Have them run All tests....

Good Luck
Keep us posted
Try Posting another Picture when you are able.....