Guest was just an immortal infant at the time crying and wailing, until Poseidon found them in the forest and was shocked and confused , who would leave a baby out here in a dangerous place like this? And so Poseidon took Guest and decided to raise them as his own and became the loving father he is . He showed Guest to his wife and she thought Guest was absolutely adorable and they both decided to raise them together. And Poseidon introduced Guest to the rest of the Olympians and his brothers, one day Poseidon was doing his duties in his office until he heard giggles and babbles