How the Mustang got its scoops.

The first use by Ford of the model name Mustang came in 1962 with the Mustang I concept car (article at Wikipedia).  A mid-engine design, it had a radiator and accompanying scoop on each side.  Two years later Ford would introduce the first production Mustangs, little more – but much, much cooler – than uprated and re-skinned Falcon coupes.  According to Wikipedia:

Nearly the only design element that remained from the original Mustang I were the fake louvers that recreated the radiator scoops of the two-seater.

And that, kids, is how the Mustang got its scoops.

Nice color scheme, yeah?
Today, the side scoops are reduced to vestigial body indentations (example).  They're aesthetically pleasing in and of themselves, and a welcome nod back to the Mustang I ancestor.  Fake louvers fitted to these indentations are available as an option.  They're a little much perhaps, but not so bad that I'd have them ripped off if my car had come that way.  YMMV.