Phoebus' people are an extinct civilization that was wiped out by Thalan's people presumably several thousand years ago.
Both cultures had fought against each other for many years. The only survivor of this war was Phoebus, who was kept alive in a Stasis pod in the Kohal system after a battle with Thalan. After being brought to Atlantis in 2006, Thalan and Phoebus sought to continue this war for their people. However, both sides lost as their bodies had failed them. (SGA: "The Long Goodbye")