Porsche 911 GT3 (996) In Gran Turismo 3. The only Porsche to appear in the series (albeit informally) so far. This car was removed entirely in the PAL version of the game. The car was likely made unplayable (and later removed) due to copyright reasons.
At the time of the game's release, Kazunori Yamauchi, creator of Gran Turismo, owned a 911 GT3 as shown here. So the addition of the car in the game might've had an original intention for that players could drive Kaz's own personal car. That's a shame, dammit EA!