Shadow Beasts, also known as Twilit and Twilight Messengers, are enemies from The Legend of Zelda: Twilight Princess. These beings, members of the Twili race, were corrupted from the darkness by the usurper Twilight King, Zant. When Zant usurped the throne, he sealed away the Sols, mystical magical orbs emanating a sacred light that banishes evil, choking the last bit of light out of the Twili and turning them into Shadow Beasts. They serve as Zant's main attack force during the invasion of Hyrule. Princess Zelda also mentions that he was using the monsters to search for Midna.
They are black, tentacled beings whose bodies are covered in a strange Twili pattern. They possess a mask similar in appearance to a shield, concealing their faces; however, the true purpose of these masks is unknown, although it is likely they are common helmets, since Midna at first uses the Ordon Shield Link finds by wearing it on her face. They are able to travel to Hyrule by way of Twilight Portals created by Zant. The beasts normally attack in groups of three to five, during which they project unbreakable walls composed of Twilight around their opponents to prevent their prey's escape. Barnes believes that those killed by Shadow Beasts in Twilight-covered areas become Shadow Beasts themselves, although this is never seen during the game. Unlike normal Twili, Shadow Beasts are able to inexplicably exist in the Light World, such as during the battle on the Bridge of Eldin, or when encountered at the top of the Arbiter's Grounds. This may be because they are protected by Zant's power, as Zant himself is able to exist in the Light World, or because these Shadow Beasts were originally people from the Light World, though this is unlikely, as it is stated by Midna they are transformed Twili.
If Link defeats all Shadow Beasts in a group but one, this remaining Shadow Beast will revive its fallen brethren with a piercing cry. In order to defeat them successfully, Link can transform into Wolf Link and use Midna's help to damage all enemies within a small radius. He can also defeat them with a carefully timed Spin Attack. Often, two Shadow Beasts attack Link in tandem while another can be found just out of Link's range, often blocked by a wall. The easiest way to defeat these is to either lure all the Shadow Beasts into the vicinity of a Spin Attack or assisted Midna attack, or to defeat the lone Shadow Beast first and then defeat the other two with a Spin Attack. After a group has been successfully defeated, the Twilight Portal from whence they came can be used for Link to transport himself around Hyrule. Interestingly, the first time Shadow Beasts are battled, which serves as a tutorial introduction to Midna's energy field power, the battle cannot be won unless Link uses the energy field to defeat the last Shadow Beast.
After Link assembles the Mirror of Twilight, opening the way into the Twilight Realm, he restores the Sols to their rightful shrine, which allows light back into the realm, returning most of the transformed Twili to their rightful forms. At two points in the game, Zant is accompanied by a pair of Shadow Beasts with different masks than the typical ones, looking more metallic and bearing a double-helix design on the masks. Because they are so few in number and appear to be Zant's bodyguards, they may perhaps have more powerful abilities than other Shadow Beasts. This is uncertain, as they are never fought in the game, and appear exclusively in cinematics. It is presumable that all the transformed Twili were returned to their former self upon Zant and Ganon being defeated.
The method by which Shadow Beasts are created is only lightly alluded to and is never shown, leaving the exact process a mystery. However, it is possible that the masks they wear may have something to do with their transformation. When Link visits the Twilight Realm for the first time, there are multiple docile Shadow Beasts standing around, and when Link regains the Sols, they revert to Twili; upon doing so, however, no Twili is ever seen wearing a mask, except for Zant. This is not entirely clear, as only a small group of Twili are shown, nor does it preclude the masks from also serving as shields, but it provides a potential mechanism by which the beasts are created and reproduce.
Behind the scenes
Shadow Beasts are also known as "Twilight Messengers", which is the name the Official Prima Strategy Guide uses. However, the official Twilight Princess website as well as their trophy in Super Smash Bros. Brawl confirms their name to be "Shadow Beasts".
A Shadow Beast shows up on a trophy, and is also featured as a sticker. The sticker gives a +28 bonus to darkness-based attacks. Like all stickers that grant bonuses to darkness-based attacks, it can only be applied to Ganondorf.
Both Midna and Zant's Standard Outfit (Master Quest) is a Shadow Beast Recolor of their standard outfit. Additionally, Midna can summon wolf-like Shadow Beasts to for her to ride and attack as part of her Shackle (Midna) & Mirror (Twili Midna) movesets.