Joseph Priestley of Thorpe Lane, Middleton, Leeds, was found guilty causing unnecessary suffering to the horse, by Penrith Magistrates Court on Wednesday (13 January).
The RSPCA found the horse tethered on a verge on 6 June 2009, during the annual Cumbrian event.
The owner could not be found so the horse, now named Vincent, was assessed by a vet and taken to Redwings in Norfolk, where he has made a full recovery.
It is thought he had been involved in a fight with another stallion.
Speaking after the case, RSPCA inspector David Holgate told a local paper: “The horse had horrific injuries and the person responsible should have sought veterinary attention.”
But he said conditions at Appleby Horse Fair had improved over recent years.
Priestley denied the charge against him but was found guilty in his absence. He will be sentenced on 28 January at Carlisle Magistrates Court.