The TIE viper was the intermediary step between TIE Advanced x1 and TIE Defender. This unique vessel carried many advantages over the standard TIE including Hyper drive, long range communication and sensors and additional deflector shielding for the main hull.
That actually looks like one of the official designs from any of the various media. I even looked it up on Wookiepedia, and I was surprised that it is in fact not listed under any variation of the Tie fighter. Excellent design, even for what may very well be a "mod" on an existing set.