The Galactic Hen is a chicken-like alien. She's dangerous and somehow smart enough to take Agent Brains's gun away. Though being or feeling trapped makes it become very docile.
She first appears in Episode 15 where she attacks a cargo ship and Brains tries to catch her. Later she leaves eggs throughout the city on earth and the gets captured by Brains again and Fred and Friday pick up all her eggs before they hatch.
She reappears in Episode 45, which wants revenge on Brains along with other aliens. But it is trapped by Friday before he could catch Fred.