I had a tamed horse and a leash, I clicked the horse with the leash and the leash disappeared from my hotbar, I then went to click a fence to tie the horse up but it would not work. it turned out I need "2" leashes in my hotbar to tie the horse to the fence, the first one gets used after clicking the horse and the second stays in my inventory after clicking the fence, but without the second leash in my inventory I could not tie the horse to the fence at all.