Are we certain that it didn't exist though? Unicode only got emoji in 2014, after ~5 year standardization effort. There were many different, incompatible formats around for about a decade before that, plus non-emoji like Kaoani.

Perhaps there was a seahorse somewhere that never made it to Unicode.

Or maybe people are just misremembering - perhaps mistaking the emoji for a unicorn or a chess's knight piece as a seahorse.