How do you get the monstrous nightmare in wild skies

First, make sure you have everything you need by checking the Nightmare's entry in your dragon book (press enter). You may need to buy the shears from the village shop in order to get the flowers.
Take the Hideous Zippleback to Unlandable Cove and ride it to Lava-Lout Island. Land, then run through the series of arches to the left.