The horses are completely fine. They arent hurting anyone.
Your horses are killing people from the land!
You're lightning bolts are killing the innocent!
How dare you!
This is impossible!
I will not be treated like this!
I shall flood this canyon to be a river.
Zues was angered that Poseidon was accusing him of his lightning bolts of hiting innocent people. This angered Zues that he blew up a canyon. Poseidon noticed this and decided to flood the canyon into a river.