They needed a plural because the "wandering hero performing Great Deeds" is found all through Greek, Roman, Celtic, Middle Eastern and German mythology, and every one of them that the Greeks heard about, they identified as Herakleis. There was a Herculotofthem.
For the same reason that you can say "two Boris Johnsons" or "two Donald Trumps". The grammatical rules have to exist for other purposes, and you can apply them to words, even where it doesn't make sense. Or it might, just in niche circumstances - "I got hit so hard on the head that I began seeing double...it seemed like there were two Herakleis!"
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