Simple. Borrow the neighbors horse, to make 18. 18/2 = 9; 18/3 = 6; 18/9 = 2. 2+9+6 = 17. Give the guy one back.
Problem is – you didn’t divide 17 horses and give a 1/2 to the oldest, nor a third to the second, nor a ninth to the third. You gave a fraction of 18!!!