Percy has a brother named Tyson, who is a cyclops. Though Percy is related to virtually every character and creature in Greek mythology through his father Poseidon, Tyson is one of the only beings whom he acknowledges as family. Percy first met Tyson at a school called Meriwether Prep, and did not learn of their relationship until much later. Percy is also distantly related to horses, pegasi, some other monsters, and numerous gods and demigods through his father. Significant examples of this would be that he is technically a grandson of Kronos, nephew to Hades, Zeus, and numerous other Olympians as well as the half-nephew of Chiron.

is percy jackson the only son of poseidon?
no