That is a good question. There were several springs on Mt. Helicon, and I suspect the answer is connected to them. I found an interesting suggestion under "Hippocampus" in Wikipedia:
The appearance of hippocampi in both freshwater and saltwater is
counter-intuitive to a modern audience, though not to an ancient one.
The Greek picture of the natural hydrological cycle did not take into
account the condensation of atmospheric water as rain to replenish the
water table, but imagined the waters of the sea oozing back landwards
through vast underground caverns and aquifers, rising replenished and
freshened in springs.
Thus it was natural for a temple at Helike in the coastal plain of
Achaea to be dedicated to Poseidon Helikonios, (the Poseidon of
Helicon), the sacred spring of Boeotian Helikon. When an earthquake
suddenly submerged the city, the temple's bronze Poseidon accompanied
by hippocampi continued to snag fishermens' nets.
Poseidon was god of earthquakes and horses as well as the sea. If Helicon had suffered tremors before it was submerged, that could explain the dedication to Poseidon. The spring at Helicon was supposedly created when Pegasus's hoof struck earth, or on the advice of Posedion himself, to stop its rising to heaven.
Two other springs there, the Aganippe and the Hippocrene, both have the word hippos, horse, in their names, which also suggest a connection to Poseidon.
