Hyakujuu Gattai GaoKing (百獣合体ガオキング, "Hundred-Beast Fusion GaoKing," also called Seirei Ō / "Spirit King" in the show) is the first and central mecha of Hyakujuu Sentai Gaoranger (2001), appearing as early as Episode 1 as the standard combination of the team's five core Power Animals. In Power Rangers Wild Force (2002) it is known as the Wild Force Megazord. Summoned as reverent, ancient beast spirits, GaoKing is the heart of the entire Gaoranger arsenal — the default robot from which the show's whole modular system unfolds.

The base formation is built from five Power Animals: GaoLion (GaoRed) forms the upper torso, GaoEagle (GaoYellow) forms the head, GaoShark (GaoBlue) forms the right arm, GaoTiger (GaoWhite) forms the left arm, and GaoBison (GaoBlack) forms the legs and hips. It wields the Fin Blade shaped from GaoShark's tail fin and strikes with Shark and Tiger attacks; its finisher, the "Heaven and Earth Roar: Animal Heart," unleashes a concentrated energy wave from the mouths of the assembled beasts.

GaoKing's standout feature is its modular combination system — the defining gimmick of the entire Gaoranger toyline. The arms and legs can be swapped out for additional Power Animals (including GaoGiraffe, GaoElephant, GaoBear, GaoPolar, GaoRhinos and GaoMadillo), producing alternate formations such as GaoKing Spear and GaoKing Sword & Shield. Beyond that, the Power Animals serve as building blocks for larger robots across the series, among them GaoMuscle, GaoIcarus, GaoKnight and GaoKentaurus — virtually every later combination is built on this core.

The DX toy launched in 2001 from Bandai Japan as DX GaoKing, featuring die-cast content for satisfying heft and the plug-together "Joint Gattai" connection system that assembles the figure from the individually- or set-sold Power Animals. For the Western market, Bandai America released the same tooling in 2002 as the Deluxe Wild Force Megazord. An absolute cornerstone of any Gaoranger/Wild Force collection: without GaoKing, none of the spectacular expansions are possible.