If the first creature targeted by Hunt Down can’t block the second targeted creature (for example, because the second creature has flying and the first doesn’t, or because both creatures are controlled by the same player), the ability does nothing and the first creature is free to block whichever creature its controller chooses, or block no creatures at all.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.