Densetsu is a Game Boy game, released in Japan(DMG-ZOJ) and China(DMG-123 CHN). The game is set during Century Zero, the main battle/war between two rival nations: the Helic Republic and Zenebas Empire. Their main weapons are Zoids and the battle/war takes place on Planet Zi.
The Zenebas Zoids were mostly red and silver and more armored, the Helic more skeletal and favoring blue and grey, it is your duty to either choose the side of Good(Helic Republic) or Evil(Zenebas Empire) and finally decide the end of the battle/war.
(In real time, Helic Republic wins and the Zenebas Empire disbands then later reforms as the Guylos Empire.)
Time to choose... Zoids of the Empire: Iron Kong, Death Saurer and Great Saber Zoids of the Republic: Shield Liger, Mad Thunder, and Gojulas.
Once you have chosen, you begin the battle/war against the opposing side and fight until the end of the war, facing tons of enemies and bosses in an attempt to win.
Source: Wikipedia, "List of Zoids video games", available under the CC-BY-SA License.