Crab

It patrols the ocean floor, walks and hops in a straight line, and does not care in the slightest whether it is underwater or up on dry sand. Nothing about the encounter is dangerous. What makes it irritating is weight: it is light enough that almost any weapon punts it several tiles away, so the fight becomes you jogging after it down the beach to land the next swing. Low knockback weapons, or anything that pierces, finish the job where you stand. Expect to meet several on any early supply run to the ocean, since they hold the shoreline before you have anything worth calling a weapon.

A Crab Statue supplies as many as you want, which is the sane way to earn the banner. Statue crabs pay out nothing, so no coins and no Shrimp Po' Boy from those, but banner progress counts the same. They also make a decent wiring component. Pen a few over pressure plates and their constant shuffling drives a circuit with no timer attached to it.