The Monster Forces is an enemy faction that appears in Hyrule Warriors and Hyrule Warriors Legends. As its name implies, the Monster Forces is made entirely of monsters and beasts. Unlike other factions, they do not have a clear leader and tend to follow other strong monsters and bosses.
Linkle's Tale: The Other Hero
Following the defeat of their leaders by Ganondorf's Forces, the surviving Monster Forces are absorbed into Ganondorf's army. In Linkle's Tale: The Other Hero, after Ganon's defeat, the Monster Forces lead the remnants of Ganondorf's Forces against Impa and the Hyrulean Forces who defended Hyrule Castle in the absence of Link and Zelda (who had left on a journey to the Temple of the Sacred Sword to return the Master Sword and restore the seal on Ganon's soul) who had yet to recover from the battle with Ganon. However, Linkle arrives to aid Impa and Hyrulean Forces in fighting off the Monster Forces. It is eventually revealed that King Dodongo from the Era of the Hero of Time is the leader of the Monster Forces and that his rage caused him to transform into Dark King Dodongo, granting him immunity to damage and the power to summon powerful Gibdos and Moblins. Fortunately for the Hyrulean Forces, Linkle's Compass is able to revert King Dodongo to normal, allowing Linkle and Impa to defeat it and the Monster Forces. However this would not be the last time the Monster Forces would threaten Hyrule.
Wind Waker: A New Disturbance
While investigating the recent appearance of islands and locations from the Era of the Great Sea under the orders of Princess Zelda and General Impa, Link found the pirate leader Tetra defending Windfall Island against the Monster Forces now lead by Boss Blin, a Stone Blin capable of speech. Realizing they shared a common enemy, Tetra joined Link and the Hyrulean Forces in defending the town and fighting off the Monster Forces. Taking Forsaken Fortress as their base, Boss Blin and his army of Miniblins, Big Blins, and Ruffians managed to drive the Great Bird, Helmaroc King from his roost in the fortress causing the Rogue monster to attack Windfall in its anger, though Tetra and Link manage to drive it off. Eventually, Boss Blin orders a Gohma to attack the School of Joy but it is defeated by the Hyrulean Forces. After capturing three of the Fortress' Keeps, the Hyrulean Forces fought with Boss Blin inside the enemy base. Eventually, Boss Blin and the Monster Forces were defeated, however Tetra was kidnapped by Helmaroc King who carried her off, with Link in hot pursuit.
Wind Waker: The Search for Cia
Meanwhile, Lana was trying to uncover what happened to Cia following her defeat, only to have her Triforce of Power stolen by a mysterious dark figure. Sensing Gate of Souls opening in Gerudo Desert, Lana and her Goron allies where confronted by the Monster Forces, which had reclaimed control over the Desert following Ganon's defeat. Lana's forces noticed the Gate of Souls were guarded by Big Blins. After Lana and her forces managed to close two of the Gates, Helmaroc King (now a member of the Monster Forces) appeared inside the Arbiter's Grounds which it decided to use as its new nest. While Helmaroc King was resting, Tetra managed to sneak away to the southern part of the desert, seeking aid from a confused Lana. Eventually Link and Proxi arrived and informed Lana that Tetra was an ally and friend of theirs. Angry that his quarry had escaped, Helmaroc King attacked to reclaim Tetra, however Lana and Link managed to repel the bird, removing Helmaroc King's Mask in the process. After fighting off some Lizalfos targeting Tetra, Tetra joins Lana, Link, and the Hyrulean Forces Gorons in closing the remaining gates. Helmaroc King returns to the battlefield, but is defeated by the combined power of Tetra, Link, and Lana. With the monster forces defeated, the trio is approached by none other than King Daphnes Nohansen Hyrule, the King of Red Lions. King Daphnes that the mysterious figure responsible for Cia's disappearance, the theft of the Triforce of Power, and the warping of time space was from the Era of the Great Sea.
While the Monster Forces are strong, they require a strong leader in order to be effective and their power pales in comparison to that of the Dark Forces, Ganondorf's Forces, and Hyrulean Forces (the three strongest factions in the game). They often rely on the strength of numbers, the power of their bosses (King Dodongo, Gohma, Manhandla, Dark King Dodongo, and Helmaroc King), or strong troops (Gibdos, Moblins, & Ruffians).