Buy a trap, place it in your kingdom, check if there is any monster in your kingdom, get rid of it, doublecheck and triplecheck if you don't find monsters. Chop the trap like you would a monster et voila a monster will spawn. Just make really, really sure there isn't a monster in your kingdom already, because then a monster will not spawn from the trap.
I definitely want to finish the event and have the leprechaun. I do hope it drops a gold item every time. If I end up having a horse as well, that will be fine, but not really necessary. I like the look of the river though.