This question confused me for a long while. It turns out that my initial thought was correct though. Historically "Amn't I" was correct, but it fell out of favour. In English we have very few (I'm struggling to think of an example to be honest) "mn" sounds. We have words spelt with "mn" (for example "column") but we don't pronounce the full "mn" sound. "Amn't" changed into "An't" and then further devolved into "Ain't" and "Aren't". Nowadays "Aren't" is more commonly seen, and "Ain't" is now viewed as more colloquial (so much so that I would recommend that you don't use it, it's only used in some dialects and sounds very out of place in any other form of speech). Interesting to note is that some dialects still use "Amn't", including some places in Ireland, Scotland and the north of England. The short answer is: "Amn't" was correct hundreds of years ago, but the language has morphed throughout the years and now "Aren't" is the correct and most commonly seen form. Reference: https://stancarey.wordpress.com/2014/03/04/amnt-i-glad-we-use-amnt-in-ireland/ Please note that this is a personal blog I found whilst researching this question, so this could all be wrong.

This is just an irregularity of the language. We don't have a standard contraction for "am not", so in the question "aren't I?" (and only in that question), we use the form "aren't" with the first person pronoun. Some people prefer "am I not?" because they find it strange to use those two words together. In fact, there is a word that is historically a contraction of "am not": "ain't". But it is now seen as substandard. Only in informal speech will you hear "I ain't" and "ain't I?", and it is no longer just a substitute for "am not". You'll also hear this word used to replace "is not" and "are not": "I ain't", "you ain't", "she ain't", "we ain't", "they ain't", "that guy with the weird hat ain't"...

Great question! Amn't was once correct and still is in some dialects. Looks like it morphed via amn't => an't => aren't (and also ain't). People copy what they hear (or think they hear) or read (but people make mistakes in their writing), so things spread, often incorrectly, and often without much reason. More info here: http://english.stackexchange.com/questions/29299/why-aint-i-and-arent-i-instead-of-amnt-i.