A long time ago, I saw a movie laid in Florida in which the plot called for an Indian to be so terrorized by threat of tying him in contact with a plant called 'Manchineel' that agrred to be cooperative. He really acted as though contact with the plant would be unendurable.
Today I found a site describing this plant, which is found in three counties in Florida - Miami,Dade, and Monroe. You have to scroll three quarters of the way down to find the heading The Little Apple that Makes Horses Mad :http://www.fnps.org/dade/pastnewslets/newslet911.html