"Chrysalides," from the Greek. Not a pluralization you hear in English often, if ever. The only other example I can think of is "irides" as the plural of "iris". The notionally correct plural of "octopus" is "octopodes", which I suppose might be formed from the same rule ("<vowel>s" → "<vowel>des"), but I'm hardly a Greek scholar. It seems that most Greek loanwords of this form have been loaned as plurals and back-constructed to singulars: "antipodes" → "antipode" (instead of "antipus"), for example.