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Every 50th kill of this enemy drops Anomura Fungus Banner.where you meet it
It lives in surface mushroom patches from the first day of a world, long before you have anything sensible to shoot it with, and once Hardmode starts it follows you into the underground mushroom biomes as well. Nothing summons it and no event is involved, so a cave mushroom farm is never as quiet a corner as it looks.
knockback barely registers
This one does not flinch. It breaks the early habit of stunlocking things with a sword until they fall over: you swing, it keeps walking, and you trade contact damage for the privilege. It is neither fast nor clever, but it will arrive.
Fight it at range instead. A bow, a boomerang, a yoyo or a flail all keep damage going while it closes. If you insist on melee, a slow heavy blade like the Breaker Blade with a Titan Potion running still holds ground, since its refusal to flinch is at least predictable.
trivia
Anomura is a real group of crustaceans, the one hermit crabs belong to, which is where the shape came from.