Hunt Down

• WWizards of the Coast2007-10-01
  Official Ruling
  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.

Rulings via Scryfall