I would suggest that your vet gives the pony a full oral examination. This would entail opening his mouth as wide as possible using a gag and looking inside the mouth with a high intensity light source. This will rule out the unlikely possibility that there is an obstruction or other abnormality. The teeth can then also be examined, which is the most likely cause of the problem.

The upper jaw is wider than the lower jaw, and as the teeth are constantly growing, the outside edges of the upper teeth and the inside edges of the lower teeth become very sharp and can cause ulcers on the inside mouth and tongue, causing discomfort when eating. These edges should be filed down with manual or motorised burrs. If the teeth are uneven or irregular, these can also be smoothed out using similar tools. It is always recommended that the teeth are checked annually and preventative filing can be carried out.