The Trelonians were a species.
Sometime prior to 2370, the Trelonian General Yiri had his brother executed because he allegedly was a traitor to the Trelonian government, though according to Doctor Bashir the evidence was only circumstantial. (DS9: "Profit and Loss")
This species was only mentioned in dialogue.
According to StarTrek.com, the Trelonians hailed from the planet Trelonia. [1](X)