Gouraijin is the battle robo of the Gouraigers – the two thunder-ninja brothers Ikkou Kasumi (KabutoRaiger, crimson) and Isshu Kasumi (KuwagaRaiger, navy) from Ninpu Sentai Hurricaneger (2002). Trained at the rival Ikazuchi ninja academy, the pair first appear as grim antagonists to the Hurricanegers before joining forces with them against the shadow syndicate Jakanja. Gouraijin is therefore a heavy, martial counterpart to the nimble Senpuujin – not a mere support unit but a full-fledged second robo with its own two-man pilot team. In Power Rangers Ninja Storm it corresponds to the Thunder Megazord of the Thunder Rangers, brothers Hunter and Blake Bradley (Crimson and Navy).

At the call of "Ikazuchi Gattai!" the Gouraigers' two Shinobi Machines merge into Gouraijin: the Gourai Beetle – KabutoRaiger's tank-like rhinoceros beetle – forms the torso and upper body, while the Gourai Stag – KuwagaRaiger's monster-truck stag beetle – forms the legs and lower half. Its signature is the Double Horn Cutter, a pair of lightning-charged blades modeled on a stag beetle's pincers that turn Gouraijin into a punishing close-quarters brawler. Where Senpuujin relies on shuriken and agility, Gouraijin is the team's blunt, straight-ahead heavy hitter. In PRNS its halves come from the Crimson Insectizord (rhino beetle/tank, Hunter) and the Navy Beetlezord (stag beetle/monster truck, Blake).

Its greatest moment is uniting with Senpuujin: together the two robos form the mighty Gouraisenpuujin (Thunderstorm Megazord in PRNS). The Karakuri sphere Furaimaru – which also supplies the combined giant's face – merges both teams into a single colossus. Gouraijin is thus a key building block in the series' ever-growing mecha kit, which ultimately scales up to the mightiest combination, Tenrai Senpuujin.

As a toy, Bandai Japan released the DX Ikazuchi Gattai Gouraijin in 2002 – a classic of the chogokin era with die-cast in the two beetle vehicles, rich paint applications, and the full two-vehicles-to-robo transformation gimmick. Bandai America reissued the same tooling under Ninja Storm as the DX Thunder Megazord (aka "Thunder Power Megazord," 2003), recolored in Crimson/Navy. Both DX versions dock onto their respective main robo (Senpuujin/Storm Megazord) to build the Gouraisenpuujin/Thunderstorm Megazord – a must-have for any Hurricaneger/Ninja Storm shelf. The original Japanese release is now comparatively sought-after among collectors.