Everybody knows the first muscle car was the 1964 Pontiac GTO , a working class hero, not some rich man's toy...

1949 Olds Rocket 88

Olds dominated the performance landscape in 1950, including wins in the Nascar Grand National division, Daytona Speed Weeks, and the 2100-plus-mile Carrera Panamericana. In (Belgium), an 88 won a production car race at Spa-Francorchamps [...] A husky V8 in a cleanly styled, lightweight coupe body, the original musclecar truly was the '49 Olds 88."

Jack Nerad wrote in Driving Today, "the Rocket V-8 set the standard for every American V-8 engine that would follow it for at least three decades[...] With a displacement of 303 cubic inches and topped by a two-barrel carburetor, the first Rocket V-8 churned out 135 hp (101 kW; 137 PS)at 3,600 rpm and 263 pound-feet of torque at a lazy 1800 rpm [and] no mid-range car in the world, save the Hudson Hornet, came close to the Rocket Olds performance potential..."

Nerad added that the Rocket 88 was "the hit of NASCAR's 1950 season, winning eight of the 10 races. Given its lightning-like success, one could clearly make the case that the Olds 88 with its 135 horsepower (101 kW) V-8 was the first 'musclecar'..."

Steve Dulcich, writing in Popular Hot Rodding, also cites Oldsmobile, concurrently with Cadillac, as having "launched the modern era of the high-performance V-8 with the introduction of the 'Rocket 88' overhead-valve V8 in 1949

-wikipedia

..mean while on the other side of the Atlantic....

..oh the excitement.