1. A warning to Percy: The hound was a clear sign that Poseidon was aware of Percy's involvement with the stolen lightning bolt. He wanted Percy to know that he was being watched and that he was in danger.
2. A threat to Zeus: The hound was also a way for Poseidon to express his frustration and anger at Zeus for blaming him for the theft. He wanted to show Zeus that he wouldn't be intimidated and that he was willing to fight back.
It's important to note that while the hound was a dangerous beast, it ultimately served as a way for Poseidon to communicate with his son and his brother.