Disabling horses..

I think the ones you saw at a neighbor's house may have been pets that are part of the pet adoption thing rather than wild horses. If it was one adult horse and a couple foals, that would be the case. Also, in map view, you should see a marker on the lot where they are regarding pet adoption (can't think what it says exactly). I use nraas overwatch to turn off neighborhood pet adoptions, which stops that. I did always think disabling them like you did within the game worked, but I've had them turned on most of the time for quite a while, and so maybe I've missed/forgotten something. If it isn't working, you should be able to use nraas register mod to eliminate them.