Theras was the son of king Autesion of Thebes. He led parties of Spartans and Minyans to the island of Calliste ('beautiful'), which thereupon became known as Thera. His name means 'hunter', 'chaser', 'seeker', from thera, 'hunting', 'pursuit', 'chase'.

This could denote his 'pursuit' for a territory of his own, since although Theras ruled Sparta, he did so on behalf of his nephews Procles and Eurysthenes. When these twins came of age and were able to rule Sparta jointly, Theras sought his own kingdom, and found it
on Calliste.