The RSPCA is trying to find the owners a pony found with serious injuries in Denham near Uxbridge.
RSPCA inspectors say the pony, dubbed Elf, was struggling to breathe because of a rope tied around its neck, which left deep cuts in the animal's muzzle, cheeks and forehead.
Police officers called the RSCPA to help them round up three ponies that had strayed onto the road.
Inspector Leanna Rice says: "It was clear no-one had been caring properly for these ponies because Elf had a rope halter on that had become too small as he grew.
"His worst wound was… so bad that there is a hole through to his sinuses."
Elf is currently being looked after by vets and will be operated on once strong enough.
Anyone with information about Elf or his owners are asked to call the RSPCA.