The Mad King's father was Aerys II Targaryen, the 17th and last ruler of House Targaryen.
Aerys II Targaryen, also known as The Mad King, is a figure well-known in Westeros lore for his descent into insanity during his rule. His father before him was Aegon V Targaryen, who had nine children with three wives—including Aerys II himself. As the eldest child of Aegon V and heir to the Iron Throne, Aerys II rose to power after his father's death in 259 AC. He reigned until he was overthrown by Robert Baratheon during Robert's Rebellion in 283 AC. He is remembered for being an unstable ruler whose paranoia led him to execute many of those who opposed or threatened him—actions that ultimately led to his downfall. Despite this notoriety however, it cannot be denied that he was indeed the son of Aegon V—making him the rightful heir to House Targaryen’s throne despite its eventual usurpation by another family.