Amestrius is the son of Herakles and Eone who is one of King Thespius. During his stay in the court of King Thespius, Herakles impregnated 49 out of the 50 of Thespius' daughters including Eone who he lay with one night. The result of that particular encounter was Amestrius.
"by Eone he [Herakles] had Amestrius"
- Apollodorus.Biblioteca, 2.7.8