Proximal and medial are terms that describe where on the horse they are talking about. Proximal is towards the body of the horse, medial is on the inside of the body. An exostosis is a bony enlargement like a splint.
I would say that this is likely to be a splint/bony growth at the top of the splint bone, but in towards the middle of the horse (so the left of the horse's near fore if looking at it head on)