You probably recognized that sound instantly and can likely name a few tunes or shows where you've heard it? (For example, one person identified it used in the film Sinbad and the Eye of the Tiger (1977) whenever the evil sorceress Zenobia made her appearance). Well, it's been around for much longer. An invention for a flexatone occurs in the British Patent Records of 1922 and 1923. In 1924 the 'Flex-a-tone' was patented in the USA by the Playatone Company of New York (wikiedpedia). Here's one example of the effect used in a vintage cartoon Scrappy's Ghost Story (1935), making its eerily sonic appearance at about 1:18 into the animation (and appearing in quite a few other places during the song such as 1:53 to heighten the lyrics "I'm a ghost. Whoo-oo-oooo!"):
Below are a few photographs of the vintage Halloween version of the Flex-a-tone (exact date of production unknown).
Shown below are some vintage ads... but you might be interested to know that professional Flex-a-tones style instruments are still sold in at least two different sizes. I have seen them available from a few online stores including Latin Percussion.