Hippeus is the son of Herakles and Prokris the eldest daughter of King Thespius. Hippeus is the twin brother of Antileon. During his stay in the court of King Thespius, Herakles impregnated 49 of 50 Thespius' daughters including Prokris who he lay with on the first night. The result of that particular encounter was Hippeus along with his brother Antileon.
"by Prokris he had Antileon and Hippeus (for the eldest daughter bore twins)"
-Apollodorus. The Library, 2.7.8.