Many people think that "Erotic" is just another versión of "Erotica". But they can be considered two different songs, because of the different lyrics they really have ("Erotic" has more explicit lyrics than the "Erotica" album version).
When Madonna decided to include a song in the "Sex" book (for hearing the song while reading the book), she wanted it to sound like "Erotica", which was already recorded by then. Shep Pettibone encouraged her to inspire in the stories of that book and writte the lyrics, changing the original lines from "Erotica", and makind them even more explicit. In fact, the part of the chorus was the one that she changed less. This is the reason to say that "Erotica" and "Erotic" are really two different songs. "Erotic" is not a demo versión of "Erotica", as this last was written, composed and recorded first. The real "Erotica" demo versions are "Love Hurts" (first demo versión of "Erotica" composed with Junior Vasquez) and "You Thrill Me" (which is closer to the final song we all know).
If we consider "Erotic" to be just another remix of "Erotica", we can say that it was remixed by Robin Hancock, and the only place where we can find it is in the cd-single that comes incide the "Sex" book (and in one of the promos from the "Erotica" single).

"Sex" is the title Madonna decided for the book in which she told us about her erotic and sexual fantasies. Madonna appears in most of the pictures, and she has griten all the text. Nowadays it is a very hard to find piece, and it is one of the collector items more important to any Madonna collector.
The book had also a little comic at the end of it, and it came along with a cd with just one song, "Erotic". It was exclusively composed and griten to be heard while reading the book.

1. Edit / Robin Hancock Edit / X-rated (4:26)
2. Sex Book Version (5:19)