Īśvara (Iswara) is one of the sons born of the womb of Pauṣṭī (Paushti), fathered by Puru, who was the son of Yayāti (Yayati).

Mahābhārata (Kinjawadekar)(1.94.5)
Mahābhārata Vols. 1-43 (Siddhāntabāgīśa)(1.81.5)