The charity was alerted when a member of the public spotted the piebald male cob-type pony lying down and in distress in the field between the A1 and the A1087 just outside Dunbar on Wednesday, December 12.
Scottish SPCA senior inspector John Toule drove to the scene but the animal had already died. A post mortem examination has revealed the six-month-old pony died of septicaemia and had not received the veterinary attention and proper diet he required.
SI Toule said: "The pony was found in an arable field and the gate had been left open onto the main road. He was first discovered on Tuesday, December 11 but the person who saw him assumed he had been put there due to flooding in nearby grazing fields.
"However, on Wednesday morning he was found lying on the ground and struggling to breathe and within half an hour he was dead.
"It is highly likely that he was abandoned as the owner of the field did not give anyone permission to leave their horse in there. We are treating this incident as a case of cruelty and we are very keen to trace this pony's owner.
"Anyone who recognises the description of the pony or noticed someone unloading him into the field between Monday 10 and Tuesday 11 December should call our Animal Helpline on 03000 999 999."