Alien Hornet

You will only meet these at the Vortex Pillar, after the Lunatic Cultist is dead and the Lunar Events are running. No item summons them and they appear nowhere else in the world. Portals tear open in the air around the pillar and drop them in pairs. The second source is the Alien Larva a dying queen leaves behind, since every larva matures into one of these.

A hornet left alone does not stay a hornet. It grows into an Alien Queen fairly quickly, and the queen then seeds more larvae, which become more hornets. The area feeds itself unless you take things off the belt early. They fly, so ground level arena tricks do nothing, and they close from above at least as often as from the side. Treat each one as the next thing you kill rather than something to mop up once the bigger enemies are down, because the bigger enemy is exactly what it is about to turn into.