Corvette ZL-1
Although it was kind of a production car, the 1969 Corvette ZL1 was essentially an experimental car with special, all-aluminum 427 V8 engine designed for racing. The car wasn’t advertised, and most dealers never knew it was available. It looked like a stock big block 1969 Corvette but was far from it (via Hagerty).
Only factory insiders and people well-informed of Chevrolet’s activities knew that ZL-1 even existed. Two examples are known, and the rumor is that there was one more.