"Otomatone", is it made by combining "sound" and "hand"?
What does Otamatone mean in Japanese?
Its body is shaped like an eighth note (quaver) where the sound emerges from its mouth on the notehead. This two-handed synthesizer instrument’s name Otamatone is a combination of two words – “oto” which means sound and “mato” which means “hand sound”.
While Wikipedia implies that the name's based on "otamajakushi":
The Otamatone is a synthesizer whose body is shaped like an eighth note (quaver) (it also somewhat resembles a tadpole, or a ladle, otamajakushi (お玉杓子 / おたまじゃくし) being Japanese for tadpole and ladle)
The translation of which is backed up by Wiktionary, of course, and Jisho.