Drops
where it comes from
It has no natural spawn anywhere in the world. Queen Slime produces it during her fight, alongside the crystal and bouncy variants she also throws out, and that is the only place you will ever see one. The difference is that this one flies. It picks a line and rams, arriving from whatever angle it was spawned at, which in practice is above and behind you while your attention is locked on the boss.
dealing with them
Individually they are trivial. The problem is accumulation across a fight that already demands constant movement. Anything that clears the space immediately around you, or minions left on cleanup duty, handles them without pulling your aim off the Queen. Turning to swat each one is how that fight gets away from people. They can drop the Slime Staff, so a wipe is not always a total loss.
trivia
It is the Hallow's reading of the Slimer, near identical in look and behavior, except that there is no wingless version to go with it. Its gore reuses the sprite from the Fledgling Wings.