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