Flukemon

Attacks
They patrol around. Then charging at the player, dealing damage on contact.

Once this enemy dies its two body parts will reanimate. Both of them attack the player. The top half hovers above the player and doesn't do much to approach. The bottom half will crawl quickly towards the player, even over walls.

Tactics
By the time you reach the Royal Waterways, you will likely have an upgraded nail and be able to 1-hit this enemy. Beware when they reanimate, as they can easily catch you off-guard, but thankfully you can 1-hit both body parts with little trouble.