Periclymenus

Periclymenus was an Argonaut, son of the two Greek demigods Neleus and Chloris, and a grandson of the god Poseidon. He received from his grandfather the ability to shape-shift into various animals. He was killed by Heracles at Pylos while fighting as a swarm of bees (as mentioned by Emily Zhang in The Son of Neptune), though he tried to escape in the form of an eagle.

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 since Periclymenus passed down his ability to "shape-shift". After Frank learns about his lineage, he tells Percy that they are related, much to Percy's, Hazel's, and later Tyson's surprise.