The Beastmen are a race of humanoid animals who pilot Gunmen and the antagonists of the first arc of Tengen Toppa Gurren Lagann.
They are artificially engineered by Lordgenome using a combination of human and animal DNA, resulting in them having varying humanoid appearances. Lordgenome created them to use to force humanity to stay underground and keep the human population under control in order to prevent the activation of the Anti-Spiral's Human Extermination System and bringing out the annihilation of the entire human race. As such, Beastmen are made to deliberately lack spiral power in order to prevent them from setting off the Human Extermination Program and are incapable of reproducing on their own.
They staff the Human Eradication Army, whose purpose to annihilate any humans who reach the surface so the human population numbers will not exceed one million.
After the Spiral War ended in the victory of the Anti-Spirals and the defeat of the Spiral forces, the Anti-Spirals set up extermination systems over Earth and many other planets to annihilate the Spiral populations of those planets if they ever exceeded one million. Lordgenome compromised with the Anti-Spirals and, to keep them from completely killing off the entire human population, assembled the Human Eradication Army from the Beastmen he created and used it to exterminate most of the humans, driving the remaining humans underground where their numbers would stay below one million.
However, after many years of Beastmen dominance on the surface, a formidable resistance to the Human Eradication Army formed from a group of humans who had reached the surface called Team Dai-Gurren to oppose the Beastmen dominion. Team Dai-Gurren, led by Kamina and later Simon, fought back against the Beastmen forces and eventually took the Beastmen's capital city of Teppelin, triumphing over the Human Eradication Army and ending the Beastmen's rule over the surface.
From there, the humans set up a new civilization on the surface where humans and Beastmen coexisted peacefully. Surviving members of the Human Eradication Army continued to wage war against the human settlements but they were mostly wiped out by the new human government's Grapearl squadrons.