Before MIDI guitars, before guitar synthesisers even, back in the 1960s there was the guitar organ. Basically these consisted of the guts of an electric organ, stuffed into the body of an electric guitar. Hollow-bodied electric guitars were popularly used, such as this attractive Gibson-esque semi, the MCI electric guitorgan.
Other than all the extra switches on the front of the guitar and the blocked off f-holes, the other main giveaway to the extra electronics inside the body of the guitar is the large access plate on the rear.
Note the split frets - these were actually the triggers for the internal organ sounds - when the string makes contact with the portion of the fret, a simple switch is triggered to play the correct note. Just think of the amount of wiring there must be beneath the fingerboard.
This particular guitorgan is not only in working condition, but was also modified for MIDI in the 1980s and so could be hooked up to MIDI synths. It's currently listed on eBay with a Buy It Now price of $2,850.