Every horse and pony needs ongoing consideration of their diet in the context of exercise, bodyweight and the quality of pasture that they have access to.
Recent surveys suggest that 50% of horses in the UK are overweight. If not in full work, most animals need minimal hard feed simply to supply them with adequate essential vitamins and minerals; ponies in particular have evolved to survive on limited grazing in harsh weather conditions and usually need restricted access to good grass. Beware that overfeeding can lead to obesity, Equine Metabolic Syndrome and even laminitis in extreme cases. The major feed companies have nutritionists available for advice if you are unsure what to feed your horse and how much. Weekly monitoring of weight via a weigh tape is a good idea to keep on top of unwanted pounds especially at the height of grass growth and richness in the Spring and Autumn.
If your horse suddenly starts to lose weight inexplicably, there may be an underlying problem which requires veterinary treatment, ranging from a worm burden to dentistry issues or even internal problems with the gut and liver. Such problems need investigating systematically by a vet in a logical manner.