Curse of Brahma
Mahabhish was a king from Ikshvaku lineage. He performed austerities for years like Ashwamedh and Vajapeya sacrifices thereby attaining heaven.
However, one day in the court of Brahma, all the devas and Mahabhish were present. Ganga too entered the court, when due to wind, her upper garments got blown away.
While all others lowered their eyes, Mahabhisha continued to stare. This act of Mahabhish annoyed Brahma, who cursed him to be born on earth, to repent and regain his status again.
Vaishampayana said, ‘There was once a king named Mahabhisha. He was born in the Ikshvaku lineage and was a lord of the earth. He wasalways truthful and truly valorous. He pleased the lord of the gods through 1000 horse sacrifices and 100 vajapeya sacrifices and thus attained heaven.
‘One day, the gods went to pay homage to Brahma. Many rajarshis and King Mahabhisha were also present. Ganga, the best of the rivers, also came to pay homage to the grandfather. Her garments, as white as moonlight, were blown away by the wind and immediately the masses of gods lowered their faces. However, rajarshi Mahabhisha continued to stare unabashedly at the river. Because of this, Mahabhisha was cursed by the illustrious Brahma. “You will be born on earth and then you will again regain these worlds.” The king then thought about all the kings and ascetics on earth and chose the immensely radiant Pratipa as his father. On seeing King Mahabhisha lose his composure, the best of the rivers went away, thinking about him in her mind. Along her path, she saw the divine vasus. They were crestfallen and dark with despair at having been dislodged from heaven.
Chapter 91, Sambhava Parva, Section 07, Mahabharat
Mahabhish chose Pratipa, from the kuru lineage to be his father. And thus he was born as the son of Pratipa and named Shantanu.
Pratipa, bull among the Kshatriyas, performed austerities with his wife, so as to obtain a son. Though they were old, a son was born to them and this son was Mahabhisha. He was known as Shantanu, because he was born when his father had controlled his senses.
Chapter 92, Sambhava Parva, Section 07, Mahabharat
According to the epic, Mahabhish was born as Shantanu as per the curse of Brahma and thus took birth as the son of Pratipa.