A muscle car is an intermediate sized, performance oriented model, powered by a large V8 engine, at an affordable price. Most of these models were based on "regular" production vehicles. These vehicles are generally not considered muscle cars, even when equipped with large V8s. If there was a high performance version available, it gets the credit, and not the vehicle that it was based on.
The strict definition only includes intermediate size vehicles. In reality, performance oriented intermediate size vehicles didn't appear until 1964. Before then, manufacturers took existing fullsize vehicles and added extra performance to them. Because of this, the early fullsize performance vehicles are generally considered muscle cars.
In addition to fullsize and intermediate muscle cars, a number of smaller vehicles started appearing on the automotive performance scene. These new "pony cars" and compact cars are generally considered muscle cars only if they have the top of the line performance engines and options.
Examples: Chevrolet Camaro (SS and Z28 models only), Ford Mustang (GTs and Boss only), Plymouth 'Cudas (no Barracudas)
Two seat sports cars such as the Chevrolet Corvette and the Dodge Viper are not considered muscle cars due to their high price and specialty nature. The only exception is the AMC AMX as it was relatively cheap, and was based on the AMC Javalin pony car.