A similar example is the English indefinite article "a", which becomes "an" before a vowel. It comes from Old English "ān" ("one, a, an"), which kept an "n" in all positions. From a diachronic (historical) analysis, the original "n" disappears, except if a following vowel needs for it to stay: "an" > "a". However, with a synchronic analysis people can also see it as epenthesis: "a" > "an". This is how most native speakers see it.