Fighting Falcon was the "official" name given either by General Dynamics or the AF. I can remember at the USAFA having to memorize aircraft silhouettes, names, and missions, and we called the F-16 the FF, not viper. Full disclosure: I'm not an AF pilot, never was. Left the Academy at the concusion of my first year because I knew I'd never get to UPT in the early 90s age of RIFing. My eyes were borderline non-PQ at the time. Interestingly, my cadet squadron was the subject of a 60 Minutes segment the summer after I left because of some bad stuff that happened during SERE training.
I know very well what it was called officially but it acquired the name Viper from the beginning. How does one know what they are going to get in 3 years?? I had buddies who didn't puck up a pilot slot then applied two years down the road and picked it up. To each their own but I don't buy that excuse. Anyway...
http://www.f-16.net/articles_article10.html
Viper: the unofficial nickname
The F-16 is often referred to as the "Viper", a nickname especially popular with people involved with the F-16. Before "Fighting Falcon" was selected as official name, pilots at Hill AFB, the first F-16 base, came up with a number of proposals, including "Viper".
Lt. Col. Pat "Gums" McAdoo, USAF Ret., one of the first F-16 pilots at Hill AFB, recalls the origin of the name "F-16 Viper":
At end of runway, the F-16 did resemble a cobra or something as it approached you. However, I think Northrop had already taken that name for the YF-17.
We all voted, and Viper came in really high. Seems there was a series on TV that had 'colonial Vipers' flying off of Battlestar Galactica (a term later used for the Eagle).
In any case, the Generals didn't want a plane 'named after some snake'!
Falcon was a good name, and it fit in with the motif that the Eagle had created. Sort of a little brother, but still a 'Bird of Prey'. In fact, GD had a great promo out in late 70's called "Bird of Prey", and it used the Falcon as the real world model.
Even when F-16 Fighting Falcon became the official name, Viper stuck around and became the unofficial nickname for the F-16. The name "Viper" is even officially used for the Joe Bill Dreyden "Semper Viper" award, which is awarded for excellent airmanship by F-16 pilots.