Stallion
The Stallion is a muscle car based off of various muscle cars (especially the one in GTA IV) with a 1968 Mustang-shaped body. The GTA IV version barely resembles a Mustang at all. In true muscle car fashion, it has V8 power channelled to the rear wheels.
It was first featured in GTA1, but didn't appear again in the series until GTA3. It's handling changed in each GTA. In GTA3 it was very unstable and often suffered from a lot of oversteer, in Vice City the Stallion can be used to easily perform drifts and doughnuts, while in San Andreas it is more stable.
The car was also recently seen in the second trailer for GTA IV, Looking for that Special Someone, with a new model.