Apterix are enormous, flightless birds native to the Great Mobian Prairie, and their meat is a rapidly expanding market in Metropolis Zone. Somewhere between ostriches and penguins in appearance, they lack the intelligence of Mobius' more common residents, and are essentially treated and portrayed as livestock. This doesn't mean they should be taken lightly however: getting caught in an Apterix stampede could be the last thing you ever do, as proven by one Boss Krouch. It seems the safest way to keep a herd moving is to do it from above, as evidenced by the Backwater wranglers' usage of Pterodactyls.
Rooster and Krouch are both owned herds of Apterix. Krouch was unhappy with the competition and ordered Chito to murder Rooster in order to be the sole Apterix breeder.