To return to the original question, metathesis is transposition of sounds, usually adjacent ones (ask ~ aks, bird ~ brid, wasp ~ waps being three from Old English); spoonerism is transposition of initial consonants; and folk etymology is a mistaken belief that a word comes from a similar-sounding word or phrase: and this is an example of all three.

Butterfly seems such an ugly and inappropriate word for the dear little things. My favourite is the Welsh pili-pala.