What is the plural form of 'Achilles(Achilles tendon)'?

Solution 1:

Why should you want "the plural of Achilles"? Nouns (common or proper) used as the non-final element of compounds don't usually take a plural ending; and if they do, they always take it. They don't change when the whole phrase is plural:

kitchen unit -> kitchen units.

cable provider -> cable providers.

and

glasses case -> glasses cases.

If you really want to talk about more than one Achilles, then most people would say "Achilleses", but people often aren't sure how to write it. I suspect some people would say "two Achilles" (like "two series").

Solution 2:

Google finds around a dozen hits for "two achilles are", and only one for "two achilleses are". So I would say the consensus is that the plural of Achilles is Achilles.

However pluralizing achilles is so rare that I assume most people who do it aren't copying the plural form from things they've heard, but coming up with it de novo. So I would be very heistant to call achilleses wrong.