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