Wikipedia
The Guitaret is an electric lamellophone made by Hohner and invented by Ernst Zacharias, in 1963. Zacharias also invented similar instruments like the Pianet, Cembalet and the Clavinet.
The instrument itself was not popular, and was dropped from the product line in 1965, presumably because it failed to excite the market. It was one of a number of experiments that Zacharias made converting non-standard musical instruments to modern ones. Guitarets that have survived have problems with the reed dampening system, which means that the instrument has come to be played with two hands.
Despite its obsolescence, its distinct tone has made it popular in both retro- and colourist settings, and it has experienced somewhat of a revival. It has been featured in soundtracks recently for this very reason.
The Guitaret's sound is that of a thumb piano. It is plugged into an amplifier, and sounds like an electric thumb piano.