The Mustang was hardly the "first midsize muscle car". The '64 GTO is generally credited as the first true muscle-car, and it was a mid-size.
The Barracuda, a direct competitor to the Mustang was released just a little earlier than the first Mustang, so technically it was the first of its kind in that genre. The instant, massive and overwhelming popularity of the Mustang created so much fervor that the whole segment was nicknamed "pony car".
"Mustang" was the most popular, but certainly not the first.
The early 'Stangs ( 65-66) were clearly not in the same league as GTOs, SSs, 442s, GTXs, Super Bees, or even Dart GTSs.
It was never considered a candidate for "supercar" status ( the term "muscle car" came later, after the party was over in the later 70's).
The only reason the first big-block was put in a Mustang ( 67/ 390 GT) was as an answer to the upcoming new 67 Camaro big block.
The first 390 Mustangs were no match for the 396 Camaros...not even close.
If you count the first 390 GT as a "Muscle Car", then it isn't the first in that category either.
( even though a "GT" is technically a different animal than a "super/ musclecar")
By the time the Mustang could legitimately claim to be a true "super/ musclecar"( late 68s with 428 CJs), the field was saturated with all manner of "supercars" from nearly every manufacturer.
Unfortunately for us...the true enthusiasts of the 71-73 'Stangs...by the time the awesome BOSS 351s and 429 CJs showed up, it was all over. Nobody cared about these type of cars anymore.
Ford truly saved the best for last with regards to the "original" Mustangs...better in nearly every mechanical, performance and aesthetic sense...by the time they hit the stage, the stands were empty.