Theagĕnes in Harpers Dictionary of Classical Antiquities (1898)
A tyrant of Megara, who obtained his power about B.C. 630, having espoused the part of the commonalty against the nobles. He was driven out before his death. He gave his daughter in marriage to Cylon (q.v.).
Theagenes of Megara in Wikipedia
There are merely a few references to the life of Theagenes of Megara amongst the ancient authors, which makes outlining a vague biography almost impossible. What we do know is that Theagenes of Megara was among the first of Greek tyrants, possibly inspired by Cypselus of neighbouring Corinth. Aristotle's Rhetoric mentions that Theagenes of Megara asked for a body guard. He states that "he who is plotting tyranny asks for a body guard." He is compared with Pisistratus, "who when granted it [a body guard] became a tyrant", a possible insight into how Theagenes managed to gain control of Megara and also insight into how the Greek concept of tyrannus might be linked with a body guard.
He slaughtered the flocks of the rich, as Aristotle mentions in his Politics. Prior to mentioning Theagenes' slaughter, he gives some insight into why this may have occurred: "They would do this because they had the confidence of the people, a confidence based upon hostility to the rich." This is paralleled again by Aristotle with Pisistratus' leading a revolt of dwellers on the plain. Aristotle mentions that military men aimed at tyranny, this might hint that Theagenes might have been a general by profession who could be paralleled with Pittacus the Mytilenaean general-turned-tyrant.
Thucydides also states that Cylon, a victor at Olympia, married Theagenesí daughter. After Cylon had consulted the Delphic Oracle, the gods told him to seize the Athenian Acropolis. It was from Theagenes that he obtained a force. He tried about 630 people in the courts to help his son-in-law Cylon get to power in Athens Cylon succeeded "with a view to making himself a tyrant.
He built a fountain house that can still be seen off the "Road of the Spring-House" in modern Megara. This fountain was built in around 600 BC, helping us put a rough date to date the time of Theagenes' tyranny. This spring is said to have the water of the Sithnidian nymphs running through it. Pausanias mentions this fountain is "worth seeing for its size and ornament and the number of columns." Athenagoras of Athens stated that Theagenes had been deified by the inhabitants of Thasos, despite his having "committed murder at the Olympic games". Plutarch mentions that after Theagenes' casting out, the Megarians enjoyed a conservative government for a short while. However, Plutarch does not expand on the nature of the exile.
Theagenes is also mentioned in Aristophanes' Peace. When the chorus are persuading Trygaeus not to sacrifice a fat swine because they would be associating with the 'swinishness' of Theagenes.