Thistle is a common name for a plant which is distributed throughout north america. It can appear in unmanaged pastures, or pastures which are overgrown. While horses and cows may help distribute the seeds, they are not a specific cause of thistle, which occurs naturally. Keeping your pastures cut and managed will help prevent thistle overgrowth. Some kinds of thistle are poisonous, and some kinds are just irritating to the mouth if consumed. Make sure you identify the kind of thistle so as to be able to determine how agressive you should be in eliminating it.
Horses and cows generally avoid it, but when alternative forages arent available, they may consume it.