Good Vibrations

That iconic wail in the background of the Beach Boys's's's seminal hit, Good Vibrations, is NOT a theremin.
I just always assumed it was-- I mean, what else could it have been?

It turns out it was a similar instrument, called the Tannerin.  Created by trombonist Paul Tanner to mimic the theremin but give greater ease of control to the player, it consists of a sine wave generator in a box with a slider to adjust frequency (clearly inspired by the trombone's slide) and a volume knob.  No dramatic waving of arms for this one.

Even more interestingly, this instrument was a complete one-off.  Only the prototype was ever made, and while it was used for several well-known songs, Tanner scrapped the idea and sold the prototype a few years later, deciding that new developments in synthesizer technology was making it obsolete (the Moog hadn't quite arrived yet, but the musical writing was clearly on the wall).

Oh, and the theme from the Original Star Trek also didn't use a theremin, even though it should have-- it was mostly heavily re-mixed vocals.