Grottu first appeared during the monster era of Marvel Comics (or Atlas Comics as it was then known), just before the dawn of Marvel’s superheroes.
Grottu was a giant and intelligent ant, born as the result of soviet nuclear tests in North Africa. Grottu could telepathically dominate other ants, and organised huge armies of ants, with which he wreaked havoc across Africa, killing animals and human’s alike, destroying whole villages and cities, and planning to “conquer mankind”, Grottu named himself the King of Insects. However Grottu’s plans are foiled by Frank and Lynn, two men from America investigating tales of the monster. They trap the monster in house, before coating him in sugar, which causes his army of ants to swarm over him, unable to control their instincts. Soon his own army either smothered him, or ate him alive (a grizzly death either way). Without a leader the ants became peaceful once more.
However that wasn't the last of Grottu. Decades later it was explained that Grottu was actually created by the Deviant Kro, who posed as a Soviet agent, and who conducted nuclear tests with the hope of creating giant monsters. Grottu was one of his success stories. It was also explained that “Frank” was actually Ulysses Bloodstone, an infamous monster hunter.
Grottu has returned several times since, presumably meaning he didn’t die in his first appearance, facing the likes of the Fantastic Four, the Beast, Iron man, and even telepathically dominating Ant-man.
Currently Grottu is imprisoned in the Negative Zone, alongside many other marvel monsters.