Shriekers were robots that resembled flying Manta Rays. They were created by Flint and manufactured on the "Planet of the Lost" for the purpose of stranding organics on the planet below.
The Shriekers were originally programmed to target technology. They were to cause space ships to crash on the planet. However, Flint later programmed them to go after organic life and had them attack Team Lightyear and the settlers.