Just as the name suggests, the keytar is a hybrid of the keyboard and guitar. Like a guitar, the keytar is worn over the shoulder and resembles a guitar in its basic form, except instead of strings is has (predictably) keys. It basically does the same thing as your stock standard synth keyboard, except it is portable. This means that just like the synth keyboard, it is an extremely versatile instrument capable of making practically any sound.
While it may not give the wielder the same instant rock star look as the guitar, that has not stopped a few metal bands have employing the use of a keytar. Among the most notable keytar toting metal bands are Dragonforce and Alestorm.
Frontman of pirate metal band Alestorm, Christopher Bowes uses it to give them their signature seafaring sound as well as matching it to their wacky sea-shanty inspired vocals, while Vadim Pruzhanov of Dragonforce is known for his long and impressive keytar solos.