To make your cob chunky you need to devise a feed & exercise regime suitable for age and fitness. However, first of all you need to look at the animals health. If the horse has suddenly dropped weight for no apparent reason I would get the Vet to run full spec bloods including liver function. I would do an egg count for worms and a blood test for worms... resistence to chemical wormers is increasing. I would have the horses teeth checkeed as dental problems can lead to condition falling off. Having eliminated any medical reasons for the weight loss I would start to increase feed and exercise. I have a lot of sucess with a high fiber, low sugar low starch diet with oil for condition and energy. I would also feed ad lib hay to keep him warm which will help him hold condition. Aim for topline rather than fat. Lots of hill work and working long and low. Hill work gives them 'bunns of steel' and a topline to die for. Simply corning them up on a load of cerials can simply lead to fat and unpredictable behaviour. Don't forget to check the fit of your saddle as your horse changes shape. I haven't met a horse yet that doesn't go through a few fittings during the muscling up process.