This happens more commonly in dogs with a mass in their chest: mediastinal lymphoma or mesothelioma (but is still pretty rare). In dogs bisphosphonate drugs like pamidronate are used to treat the new bone deposition, but this would likely be cost prohibitive and impractical in a horse. I would LOVE to see chest x-rays on that mare!!!
Yea, I have never heard of it in horses but it is "common" (or "not uncommon" ha ha) in dogs. Chest tumors cause these weird bony formations in the limbs, and as soon as you remove the chest tumors the bony formations go away. BTW is Tildren considered a bisphosphonate? There is another one that starts with a Z- know what I am talking about?