Thyresta are an extinct alien species from Orion's Arm.
The Thyresta inhabited the planet known as New Montana (YTS-4885002-3825-IV; MPA). The Thyresta were endoskeletal four-limbed creatures. They were equipped with two horn like structures ending in manipulators, weapons and social signals. They evidently lived in complex familial nests, before wiping themselves out.
The Thyresta wiped themselves out in a nuclear war, about 3.6 million years ago. Before this, they were an advanced, sophisticated civilization. Sometimes they are known as "Mankind's Contenders" because if they had not destroyed themselves, cultural evidence suggests they might have become a dynamic advanced and expansionistic civilization like mankind. They are the most recent example of a race that failed to avoid the risks associated with advanced technology; there are older ones, but the remains of which are not so well preserved. If a species develops high technology and but uses advanced weapons before interplnetary travel is developed, they will likely share the same fate as the Thyresta.
A related species, Thyresta Minor, is possibly a subspecies or devolved descendant of the Thyresta remains and are being uplifted by MPA organizations on the orbital habitats surrounding New Montana.