Poseidon: Earth Shaker (Olympians #5)

Poseidon, the god of the ocean, is a wrathful as a tidal wave and sullen as the ocean's stormy depths. But he cares tenderly for his children, monstrous though they are. The winged horse Pegasus, the merman Triton, the one-eyed giant Polyphemus, and perhaps the worst of them all, the hero Theseus... Mighty Poseidon loves his sons.