"Demons"were a montrous Galaxian fighters that assaulted the Solar System.
The term "Demon" is by no means they're real title. The tag was given to them by AGAD after a captain in a squad attempting to hold them off asked:"What are these montrous Demons?." The captain was killed trying to defend a rookie in the squad not long after, so they gave the name to the galaxian fighter not long afterwards. The Demons were armed with Alpha Missiles and Energy Projectiles. The Demon was also known to be extremely fast. Since they were still in prototype stage when they were destroyed no more of them exist.