Shadow Beast

Shadow Beasts (影の使者, Kage no Shisha: Shadow Messenger), also referred to as Shadow Beings, Black Beasts, or Twilit Messengers, drop out of Portals all across Hyrule and in the Twilight Realm. They are Twili that have been corrupted and turned into monsters by Zant. Shadow Beasts are strong enough that only a few were needed to overrun and defeat the many Hylian Soldiers guarding Princess Zelda, despite the fact that the latter had advantageous numbers.

If there is ever only one standing Shadow Beast during a group battle, it will utter an unearthly howl that rejuvenates the others. Thus, at least the last two must be killed at the same time in order to truly defeat a pack. Once there are only a few left, most area attack can finish them off, though some will not function correctly, such as the Jump Strike or the Mortal Draw.

Different Shadow Beasts are seen in the cutscene which shows Zant taking Hyrule Castle and Midna's history of the Twili. These are larger than the normal ones, and bear a different head-dressing; a round silver plate with a special marking on them. However, these monsters are never seen during actual gameplay, so their status and any exclusive powers remain a mystery.

Trivia
An early beta version of Twilight Princess included what may have been a Shadow Beast in the very early stages of development.