You certainly don't hear of many horses on the line. If there are animals on the line it seems to be cows or sheep normally. Although i think the strangest one i have heard of is llamas on the line between Hildenborough and Sevenoaks in Kent a few years ago.
Llamas are not unknown. Ditto Alpacas. Had an Ostrich on the line on the East Suffolk once. Also a dinosaur on the line on the GEML, albeit a large fluffy stuffed one.
