F-20 was the renamed F-5G. The F-21 was a small batch of Israeli Kfirs on loan for DACT out at Nellis.
Planes such as the F-11 did in fact exist, but no one really knew them by that name. They were by and large 50's planes such as the F11F or F5U that were given new designations with the 1962 reorganization that occurred when McNamara got pissed off that the USAF came in talking about their new F-110 and he later found out it was the SAME plane as the USN's F4H!
Here's a quick list:
F-10 -> F3D Skynight
F-11 -> F11F Tiger
F-12 -> Actually the YF-12, it was an interceptor based on the A-12 used by the CIA, the early production variant of the SR-71 : http://en.wikipedia.org/wiki/Lockheed_YF-12
F-13 -> pilots are a superstitious lot. Who'd fly an F-13?
As for the YF-17, it was a very different animal from the F/A-18A that came from it. There's at least one picture out there of the 2 parked next to each other. The 17 was as noticeably smaller than the Hornet than the Hornet is when next to a Super Hornet.
I'd love to see a Super Hornet next to the 17.
As you can see here: http://www.aerospaceweb.org/question/planes/q0145a.shtml
http://airpower.callihan.cc/images/xyplane...2WMF-YF17-6.jpg
the YF-17 differed quite a bit from the Hornet we know now.
Interesting thing about the 2nd pic, from the Western Museum of Flight in Hawthorne, CA, is it's sitting next to one of the 2 YF-23 prototypes.