Shadow Beast

Shadow Beasts, formerly known as Twilit Messengers, are monsters created from Twili altered by Zant's evil magic. They have large black bodies with a gorilla-like stance and a large shield over their heads. Zant uses them as his primary elite troops.

The Legend of Zelda: Twilight Princess
In The Legend of Zelda: Twilight Princess, Twilit Messengers are not formally named at all in-game, simply being generically referred to by various phrases such as "shadow beings," "shadow beasts," and "black brutes." However, the official website calls them Twilit Messengers. They are created from Twili that have been transformed by Zant, though a line from Barnes indicates they may also be able to convert spirits into more of them. They were used by Zant to storm Hyrule Castle and overwhelm its soldiers, forcing Princess Zelda to surrender to Zant and allow him to cover all of Hyrule in twilight. They also scout Hyrule in search of Midna. The first Twilit Messenger to directly appear pulls Link into the twilight-covered Faron Woods. It is then briefly incapacitated after Link's Triforce mark glows, but after Link turns into Wolf Link and passes out, the creature drags him to Hyrule Castle's lockup.

Twilit Messengers can appear automatically on the field when in twilight and drop from red-marked portals when outside it. Whenever fighting a Twilit Messenger or group of them, the area is surrounded by walls of twilight magic. They can be attacked in various ways and take a moderate amount of hits to fell. If only one of a group remains standing, it shrieks to revive the others. In order to get around this, he must defeat multiple at once, such as through a Spin Attack or through Midna's capture field ability as a wolf. When an encounter has been completed, they turn into twilight squares and form a cyan-marked portal in the sky. Link does not fight a Twilit Messenger until receiving the Ordon Sword and Ordon Shield, where one ambushes him in his wolf form at Ordon Spring. A group battle occurs shortly thereafter in Faron Woods and specifically requires Midna's field attack to complete, acting as a tutorial for it. They continue appearing throughout the game, generally to create shortcuts through the portals upon defeat.

In the Palace of Twilight, Twilit Messengers appear as normal enemies unlinked to each other and without portals. Twili in the process of transforming can be seen outside and saved by placing the sols back in place. Once Zant is defeated, no more Twilit Messengers appear.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, a Shadow Beast makes a small appearance as a trophy.

Trivia

 * During certain flashback scenes involving Zant, he is flanked by a pair of unique Twilit Messengers with smooth, circular shields with a helix design and angular protrusions. While models for them exist in the game's data, they are never faced.