For centuries, Relicanth, like other well-known finds such as Armaldo and Omastar, were seen as a textbook example of an extinct pokemon. When the first were captured in the modern era with the advent of deep-sea fishing, many remarked that this pokemon had long been unknown to science. Yet unknown to science need not mean unknown to history; merely poorly documented enough that it was mistaken for myth.
The creators of the legendary golems, the so called "blind faith" community of Mount Coronet, were undoubtedly well aware that Relicanth existed, for they chose it and Wailord to be the twin keys to their automaton guardians. Records this community left behind show undoubtedly that they were aware of this pokemon, and explanations which rely on their awareness of fossils faltered after highly-publicized attempts to open up the gate with a well-preserved Relicanth fossil failed.
Yet the fact that this community must surely have possessed Relicanth has raised new questions. Their links to coreligionists as far away as Hoenn and Unova explain their access to Wailord, to be sure. But the technology to reach deep into the ocean was clearly unknown even to these people who could turn lifeless rocks and lumps of ice or metal into walking guardians of their sanctuaries. Some think it was the Wailord themselves who dove deep into the ocean to catch the Relicanth, but although Relicanth are captured every day by hungry Wailord, the lavish religious art of Mount Coronet depicts no Relicanth being carried in Wailord mouths!
Perhaps the true purpose of requiring Relicanth was not to unlock the guardians but to seal them away. Their seal was meant to be broken only when man had learned to accomplish the impossible, a time which only came long after their civilization had perished.