The mythological beast called the Hippocampus was associated with Poseidon, but it's not true to say that Poseidon rode a specific horse named Hippocampus (which is what this question seems to imply).
The hippocampus was a type of sea–horse in Greek mythology, with the tail of a fish, and Poseidon's chariot was believed to be drawn by hippocampi. But there was no individual horse named Hippocampus, and as far as I can establish, Poseidon didn't ride any horse.
Poseidon is represented along with a hippocampus in Rome's Trevi fountain.
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