Ronin is what you could call the Anglicized version, I guess. Since English speakers don't know about long vowels (or Japanese pronunciation in general) words are often dumbed down when written in English. It's a pain for the language learner who comes across a word for the first time and wants to look it up, but it seems that for the most part that audience isn't considered too important. There are a few different ways to show the long vowel in 浪人 (which means "lordless samurai," or nowadays someone who's failed the university entrance examination): rounin, roonin, and ronin with either a macron (looks kind of like -) or a circumflex (looks like ^) over the "o" (which I can't get to display properly).
Just so you know, ろにん (ronin) has no matches in the Japanese dictionary, but ろうにん (rounin, roonin) does.