Periclymenus

Periclymenus was an Argonaut, son of the two demigods Neleus and Chloris, and a grandson of Poseidon.

History
Pericylmenus received the ability to shapeshift into various animals from his grandfather. When Hercules waged war against Neleus, King of Pylos, for refusing to purify Hercules for the murder of Iphitus, Hercules killed eleven of the twelve sons of Neleus, one of which was Periclymenus. With the assistance of Athena, Hercules recognized Periclymenus as a bee, settled upon the yoke of his chariot and shot him with an arrow.

The Son of Neptune
Frank Zhang and the rest of his family are descendants of Periclymenus. Every member of his direct family are able to transform into animals as Periclymenus passed down this ability. After Frank learns about his lineage, he tells Percy Jackson that they are related, much to Percy's, Hazel's, and later Tyson's surprise.