Coconut Crab, the Mother of Nopes.

The fear itself

The Coconut Crab. This fucking thing. I live thousands of miles away from its natural enviroment, and I still check under my bed every night in fear of these things. Fuck your spiders and other light-weights, THIS is the ultimate nope. It's by far the largest land-dwelling arthropod in the world, as well as of the biggest known land animal with an exoskeleton. Likely it's the largest such creature in the planets recent athmospheric history. It can weigh 4.1 kg (9.0 lb) and grow up to a fucking meter (3 ft 3 in) from leg to leg. Jesus Christ that's terrifying. Just look at it.

From the depths of hell

It lives on the islands across the Indian Ocean and on parts of Pacific Ocean, and is found on mainland of Australia and Madagascar. I'm assuming they came here from either Mars after eating up everyone in there, or Hell, after Satan got too scared of being around them.

Evil incarnate

Coconut crabs have organs called branchiostegal lungs, which they use for breathing instead of gills. They can't swim and will drown if they're in the water for too long. That is why I recommend sinking every damn island between the eastern shore line of Africa all the way up to the western shore of South America as well as quarantining Australia. We must not let evil prevail.

Worldeaters

They have an acute sense of smell which they use to find food. I'm also pretty sure they can smell my fear from half-way around the globe. Officially they feed on fruits, nuts, seeds and pith, but also "other organic material". I'd like to remind you all that humans are organic. Babies are organic. These things basically feed on babies.
They're also famous for eating coconuts. They actually climb in trees to drop them down.
They can climb. You're not safe even if you live in a top-floor apartment.

Lurking in the shadows

Juvenile crabs use gastropod shells for protection similar to Hermit Crabs, but adults develop a tough exoskeleton and stop carrying a shell around. As adults, they live alone in solitude underground and in rock crevices, stalking for humans and coconuts. They dig their burrows in damp soil and stay hidden during the day to avoid the heat. But during the night, they feast on human brains! And fruits. Fruits and brains. Mostly fruits. They also surface on rain to avoid drowning.

Wondrous death

They have no natural enemies other than themselves and humans. It's hunted for its meat and it's rare on populated islands for this reason. Its existence is threathened in some areas due to excessive hunting. And seriously, despite it being a scary-ass motherfucker, it isn't evil. It's more neutral. Likely chaotic neutral. It's harmless to us and it's actually a really interesting and wonderful animal, as long as it stays the fuck away from me.

Educate yourselves

If you want to read more on the Coconut Crab, start here: https://en.wikipedia.org/wiki/Coconut_crab

Also, I've done more of these posts. You can find them in my submissions. The previous one was the Devil's Flower Mantis: http://imgur.com/gallery/rCZnv/