To tell somebody else to "take back YOUR fuckin horse" means for that person to take back a problem they have cast upon you, without your willing consent.
When you throw or cast YOUR problem(s) onto somebody else, just to get that trash away, it will come back. The "Horse" is the problem, it can be anything that is causing issues for you, and you dont want anymore.
So to truely take back the horse, means the other person you cast it on, is fed up with having that shit on them, and they feel its time for you to deal with the problem, not just give to others.
Frank thinks: "Im fuckin tired of Betty always hitting on me, why wont that ulgy slut leave me alone!"
____The next Day___
Fred: "Hey Betty, i felt it was time for me to finnaly treat you right, so im gona hook you up with the coolest guy i know. trust me, he fucks like a tiger. His name is Tom."
Betty: "O hell ya, i havnt got laid in weeks, thanks ^_^"
Betty: "Hey tom, wanna fool around, im so horny right now"
Tom thinks: "Damnit Frank, i dont want this slut on me now -"TAKE BACK YOUR FUCKIN HORSE"-