Shadow Beast

Shadow Beasts, also referred to as Shadow Beings, Black Beasts, or Twilit Messengers, are the muscle behind Zant's twilight invasion in Twilight Princess.

Characteristics and Weaknesses
The Shadow Beasts are dark, willowy beings that drop out of Portals all across Hyrule and in the Twilight Realm, always in packs of three or more. 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. It is also hinted by Barnes that they are not only composed of transformed Twilis, but also have the ability to turn Humans into their kind, meaning that they are not restricted to one species.

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 attacks 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. One possible theory is that they are a sort of honorguard for Zant, as they always appear near him, generally in flanking positions.

Super Smash Bros. Brawl
Shadow Beasts appear as a trophy and sticker in Super Smash Bros. Brawl.

Creation
The exact method by which Shadow Beasts come into being is only slightly alluded to and is never shown, leaving the process a mystery. It is possible, however, that the masks they wear play a role in the transformation. When the Twilight Realm is visited for the first time, several docile Shadow Beasts are standing around the Palace of Twilight; once the Sols are retrieved, these beasts revert to their normal Twili forms. Upon turning back into Twili, their masks are destroyed, and no Twili is shown to wear them again. This might indicate that the masks were responsible for their transformation. It is worth noting, however, that the masks are also thought by some to be shields (as Midna tries to wear the Ordon Shield on her face once it is acquired), and Midna seems to indicate at one point that simply being in the Twilight is enough to cause some people to become Shadow Beasts. However, the latter explanation fails to account for the Twili (who had already lived in the Twilight Realm for generations without becoming beasts), while the former could hold true without necessarily invalidating the idea.

Trivia

 * An early beta version of Twilight Princess included what may have been a Shadow Beast in the very early stages of development.
 * When Link first fights multiple Shadow Beasts, if the spin attack is used on the last two, one will still survive, likely so Midna can teach Link the energy field attack.
 * The official Japanese website for Twilight Princess calls them "Shadow Assassins".
 * Their and ability to revive fallen Shadow Beasts may be a reference to the phrase, meaning "to raise the dead".
 * The Shadow Beasts' hands resemble that of the enemy Zant's Hand in terms of the middle two fingers are shorter.