Is a seahorse a lesbian symbol?

Actually, mermaids were first conceived of in ancient mythology as a symbol of heterosexual lust. Stories were told about male sailors on long voyages who believed they saw beautiful mermaids (or "sirens") basking on rocks, and sailed towards them hoping to get a better view. The mermaids' beauty would distract the sailors long enough for them to lose control of the ship and ultimately wreck on the rocks.

That said, it's definitely possible that someone or some group of people has used a mermaid as a lesbian symbol.