The Bosquito is an incredibly powerful and dangerous alien lifeform feared across the universe and the overarching antagonist in Chouseishin Gransazer. It is something the Warp Monarch seeks to eradicate. Despite being wiped out in the war that obliterated the ancient Earth civilizations, Belzeus claimed that the humans were descended from the Bosquito as a way to encourage the Warp Monarch to finish the global genocide they started long ago.
The Gransazers were originally created to counter the Bosquito when it came to Earth. The Bosquito was wiped out when the Warp Monarch destroyed Earth's ancient human civilizations in an attempt to exterminate it. However, one Bosquito somehow survived and was sealed away.
That Bosquito was later revived inadvertently by Luka under Belzeus' instructions. Luka attempted to hold it off using some Gig Fighters, but it destroyed them with help from Tenma, who mistook the Bosquito for the thirteenth Gransazer. It then knocked out Tenma when he attempted to protect Luka from one of its attacks and defeated the other two members of the Flame Tribe. It later took control of Yuuhi and hijacked Garuda when it was summoned by the Flame Tribe. Despite being ejected from it by Leviathan and Guncaeser, it was able to increase it's strength by absorbing energy from floral life and create two clones of itself.
After it's clones were destroyed, it absorbed the orbs they left behind and transformed into a giant monster. It was killed when Tarious used one of Lion's Crescents to take the ruby on it's chest, allowing DaiSazer to destroy it.