"Piranha" is a Portuguese rendering of a Tupi word referring to a sharp-toothed fish with an unfortunate reputation. The correct pronunciation of this word is something like /pɪˈɹɑːnjə/. However, the "mis-pronunciation" /pɪˈrɑːnə/ (peronna?) is commonplace, and has been for some time; at least as far back as 1972, ichthyologist George Myers's Piranha Book claimed that "most English-speaking people tend to mispronounce it".
What strikes me is that when "piranha" was introduced, presumably most everyone who knew the word would also have known the Portuguese origin and pronounced it as such. Even as the word spread to broader society, people should still have seen or heard it alongside its competing Spanish rendering, "piraña", which seems to have even less ambiguity about its pronunciation. Still, somehow the /nj/ was partly forgotten, and the Anglicised pronunciation gained market share.
How and when did this shift in pronunciation come about? Are there records of commentators earlier than Myers describing and/or bemoaning the new pronunciation? I have a half-formed hypothesis that Theodore Roosevelt's 1914 Through the Brazilian Wilderness might be partly to blame, given its titillating marketable lies about the fish's behavior and its lack of a pronunciation guide, but I don't have enough information to make a firm conclusion about that book's relevance.