Masculine: Bonjour mon bel amiFeminine: Bonjour ma belle amieboth mean Hi my beautiful friend.
Perhaps you meant Mon ami, which means my friendNo, "mon auto a moi".
That's how it is simply because it would be difficult to pronounce if the first word ended with an a, but normally you'd have to use 'ma' in front of an object if it is feminine. Normally you'd say 'ma amie' because the object of the sentence in this case the friend is feminine, but because the first word ends with a vowel and the second word begins with a vowel, you have to say 'mon amie' instead. If you're talking about a female friend, you would say 'mon amie.