Did Forbidden Planet have a theremin? I don’t think it did, and I don’t recall a theremin sound in the score.
I always thought so, but apparently not. Wikipedia says:
“While the theremin had been used on the soundtracks of Spellbound (1945) and The Day the Earth Stood Still (1951), the Barrons are credited with creating the first completely electronic film score, preceding the development of analog synthesizers by Robert Moog and Don Buchla in the early 1960s.”