Buleus is the son of Herakles and Elakhia 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 Elakhia who he lay with one night. The result of that particular encounter was Buleus.
"by Elakhia [Herakles] he had Buleus"
- Apollod. Biblioteca 2.7.8