DISCLAIMER: Taken from "Bess" by Alvin Schwartz. I don't own PJO either.
PEGASUS
Poseidon raised horses, both on land and under the sea. He had many of them, but his favorite was named Pegasus. Pegasus was a beautiful black horse that always obeyed Poseidon. Unfortunately, Pegasus could no longer support Poseidon's weight, because he was getting too old.
That summer, for the Hades of it, Poseidon went to the Oracle of Delphi. The Oracle studied her fortune-telling cards, and breathed, "I see something bad for you." She glared at him. "Your favorite horse will cause you to die. I am not sure when, but it shall happen. It's right here in the cards."
"That's a bunch of bull crap," Poseidon laughed. He knew that it was all crazy. Pegasus was as dangerous as the carpet in Poseidon's bedroom.
However, whenever he saw Pegasus, he remembered the Oracle's warning.
That fall, Demeter—who owned a farm on the other side of the country—asked if she could have Pegasus. Her daughter Persephone was living with her, and she said she'd like to ride it.
"Of course she can!" Poseidon boomed with delight. "Persephone will give Pegasus something to do."
Later, he told Amphitrite about it. "Now Pegasus won't kill me, honey!" And they both laughed.
That spring, he met up with Demeter in the store. "How's Pegasus?" he asked her.
"Oh. He was okay for a while," Demeter explained. "Persephone fell in love with him. Then he got sick, so, unfortunately, I had to shoot him to put him out of his misery. It was rather disappointing.
Poseidon let loose a huge sigh of relief. He'd wondered how Pegasus could kill him.
"Can I see him? He was my favorite. I'd like to…just say goodbye."
The bones of the dead horse were in a corner of Demeter's farm. Poseidon knelt down and touched Pegasus' skull.
Just then, a scorpion—which had made its home inside the skull—stung Poseidon and killed him.