How do I build muscle on my underfed pony?

Hi! To strengthen the back, find exercises that ask it to work in a way that helps it carry itself, and then carry a rider. Watch how the horse holds its body, including its head. Make sure the topline is long and that the horse can work in a relaxed way.

A few exercises to do from the ground:

Vary the stride length in walk. Let the horse stretch out and walk on with a long neck, free of the aids, then ask it to walk slowly, so slowly it almost halts. Wherever it is hard to keep an even flow is where you need to work more.

Walk over poles. Set a number of poles on the ground for the horse to walk across, varying the distance between them so the horse has to stretch out.

Transitions to canter. The transition itself is a very good exercise for the back, as is each individual stride. Work from the ground, ideally loose, so the horse can use its neck to balance.

Then, of course, a good feed ration matters. It needs to contain the building blocks the muscles need to develop, so a good balance between energy and protein is essential.

Good luck!