The Anomura Fungus is an enemy found in Glowing Mushroom biomes above the surface at any point in the game. During Hardmode, they can also spawn in underground Glowing Mushroom biomes, however on the version they can spawn underground at any time.
Tips
Due to their high resistance to knockback, using short range weapons, such as Swords or Spears, is not recommended. One should attack these enemies with long range projectiles that give large amounts of damage such as Bows, Boomerangs, Yoyos, or Flails.
Minions add extra damage, even from a distance, and make the fight much easier.
The Breaker Blade combined with a Titan Potion provide a close range alternative. The Anomura Fungus's knockback resistance should allow multiple hits while still keeping the enemy at a distance.
If you aren't in Hardmode, use a Titan Potion and/or a high knockback weapon.
Trivia
Anomura is a group of crustaceans that includes hermit crabs.
The enemy's appearance is a crab with a mushroom on its back, therefore the Latin word "Fungus", mushroom, in its name.
The noise it makes upon death is a low-pitched version of the Granite Elemental death noise.