Opposite of the topic (ultra-nonconserved?), "butterfly" is a word that is strangely different in even closely related European languages (Romance, Teutonic, Slavic). Doing a cursory check in Google Translate now, but I've one found one language pair where the words appear to be related: French: "papillon", Catalan "papallona". Otherwise: mariposa, бабочка, motyl, schmetterling, vlinder, sommerfugl, fjäril, farfalla, пеперуда, leptir ...
I'd love to hear a linguistic explanation for this.
EDIT: Latin: "papilionem" (papilio?), so at least French and Catalan have conserved it, and I can see that Italian "farfalla" could be cognate.
EDIT EDIT: All the Slavic languages known to Google Translate have a word related to motyl, except for Russian: бабочка (butterfly) (but мотылек (moth)), so there is less to this phenomenon than meets the eye!