If it were me, just to prove a point that I can do whatever I'd like on my own property, I'd take down the section of fence that I put up (that your neighbors apparently have assumed is their to use freely), move it over five feet, and let the neighbors build secure fencing for their stallion. Start the project ASAP, that way the neighbors won't have a fence to put their stallion behind BEFORE they put him out there. They do have a duty to ensure that they have safe and secure enclosures for their horse. Stallions are especially hazardous (ask any homeowners insurance that insures horse farms) - the burden is on the neighbors to take care of their horse. The law is definitely on your side on this, no matter what you choose. However, I would suggest taking your fence back a few feet from your property line, anyhow... just so they don't build their own right directly against yours... we personally allowed enough room on the outside of our fence for a utility vehicle to get along it (to make repairs without disrupting the horse in the paddock).
Last edited by sillyhorses; 11-04-2012 at 10:16 PM.