What is the name of the giant mecha suit built by Kuvira?

The giant mecha suit built by Kuvira is called the Colossus. It is the enormous spirit-vine-powered war machine that Kuvira deploys against Republic City during Book Four: Balance.

The Colossus is far larger than the ordinary mecha-tanks seen earlier in the series. Its armored body gives Kuvira a mobile fortress, while a spirit-vine cannon mounted on one arm provides a devastating long-range weapon. The machine reflects Kuvira’s combination of metalbending expertise, industrial control, and willingness to turn spirit energy into military technology.

Kuvira uses the Colossus to pressure the United Republic into surrender. In the episodes “Day of the Colossus” and “The Last Stand,” Korra and her allies attack the machine from several directions. Its armor and size make it difficult to stop, but coordinated sabotage eventually brings it down. The Colossus is often confused with the smaller Equalist mecha-tanks from Book One, which were developed by Hiroshi Sato rather than Kuvira.