Pokemon cries are made electronically, so it's not really an instrument that makes Meloetta's cry. However, the midi instrument that was used to create its cry seems to be a vocal synthesizer of some sort, however it's be very difficult to track down exactly what soundfont was used to create it. In a program, (TuxGuitar, to be specific) I recreated Meloetta's cry, and the three midi instruments that seemed to match the best were "Choir Aahs," "Synth Choir," and "Voice Oohs." In different programs, these will probably be called different things, but the main idea of this is that a voice synthesizer of some kind. If you're interested, here are the recreations of Meloetta's cry with the different instruments I mentioned, as well as the music.