There's nothing funny about a keratoma but German farrier Loic Entwistle couldn't resist the temptation to turn this hoof into a jack o' lantern. Or is it the face of a ghost that made this photo suggest Halloween?
It looks like this foot may be growing out after a keratoma (hornsaule in German) in the toe of the hoof wall was surgically removed, and was brought back to the hospital for a trim and clean-up of the wall defect. The veterinarian probably marked the hoof wall area to be cleaned up with a marker...and it became a face!
A keratoma is a benign or non-cancerous tumor made of horn. I think they are either more commonly found in Europe or they are more aggressive in removing them as European farrier textbooks always have lots of photos of keratomas and they even explain the different types based on where in the wall the tumor is located and describe the common shapes the tumors will take depending on location.
Even though a keratoma can reside uneventfully inside a horse's hoof wall for a long time, it can also sometimes grow large enough to press against soft tissue and/or the coffin bone, or cause chronic abscessing.
To learn more about keratomas, read "Hoof Wall Resection and Reconstruction for a Tubular Defect" by Andrew Poyntom FWCF in Hoofcare and Lameness #78, and the chapter on different types of keratomas in the book Hoof Problems by Rob Van Nassau, available from Hoofcare Books. (Click here to learn more about this book and order your copy.)
If you read German, another excellent treatise on keratomas is in Uwe Lukas's Gesunde Hufe-kein Zufall available from the German Equestrian Federation's online bookshop.
Many thanks to Loic Entwistle and his amazing photo library for the loan of this image.