A horseshoe crab is fucking gnarly, bro. You see this brown, almost crescent/spherical object with a pointy tail skuttling along, innocuously along the bottom of the shallows, and moving at a decent pace. It's so different from anything else you'd expect to see with the tides, and much larger too. Bigger than any crab, bigger than most fish that like to hang at that shallow depth. You might even mistake the two round black dimples in it's carapace for eyes, and like the comical depiction find it...cute?

It looks fairly non threatening, and has a poor reaction time. So like any kid you pick it up by it's tail, and then, "HOLY FUCK!" Nightmare fuel, as you look under it's carapace and at the many legs writhing underneath like some Cthulhu fever dream.

You put it back down in the water and let it do it's thing.

I've never witnessed it in person before, but my Mom who was an Environmental chemist for the EPA saw these things breeding once, and it's like a giant migration to the shallows and shore for these things to engage in one giant orgy. Just absolute hundreds of them coming to fuck in the name of their Nameless God.