Goliath Bird Eating Spider
It's frequently described as "larger than a dinner plate," presumably because spider-scientists don't want to freak people out with the significantly more relevant "big enough to fit over your face." It's a foot across and, when it's in the mood, brings down entire birds and eats them. It doesn't make webs, or leap great distances--it just slowly sneaks up behind things and jumps on them. Yes, just like in your nightmares. Go ahead and check behind you. We'll wait.
As if a giant spider that can bring down a moderate-sized flying animal wasn't bad enough, it's one of the "hissing bastard spiders" (that's actually its full scientific name), the creepiest creature to exist outside of a horror novel. At this point, evolution clearly just started piling in everything nasty that could possibly fit: Its spit can dissolve flesh down to the bone AND it can shoot invisible hairs that cut your eyes and mouth. While the "intelligent designer" theory has lost steam of late, the "sadistic bastard creator" theory is single-handedly supported by the existence of this creature.
Its venomous bite is not fatal, but can cause pain and swelling. We'd like to take a moment to praise those bite victims who did not die of sheer terror, and who we presume now work in Afghanistan kicking sand for landmines, having utterly used up their entire life's supply of fear. When you've looked down and seen a screaming Frisbee-sized spider with its fangs sunk in you, you're probably going to need to be thrown out of an airplane into a volcano before you'll ever feel mildly concerned again.
Completely unaltered photo. From this site.
So how do we finish them off?
No messing around here--there is an entire part of the brain designed to keep us the fuck away from spiders, the "arachnid response" to anything horrifically alien that must be destroyed. The only reason this abomination isn't already extinct is we can't find enough Navy SEALS prepared to go near the damned things. Heroes? Psh, the only reason those bastards dismantle nukes is because the "Jobs Available" list at the base reads:
a) Single-handedly engage the entire terrorist forces of Mujhadikkaklikkastan or
b) Seriously guys, one of us needs to kill those fucking spiders. But I've hurt my leg. Signed, Rambo.
Do you have an irrational, creeping fear of insects? Allow us to replace it with an entirely rational terror of them! Check this: A slavemaker queen fakes death, allowing itself to be carried into an enemy nest. There, it then rips their queen apart, coats itself in the bits left over, then immediately starts pumping out soldier eggs. The locals then raise their own enslaving army to maturity.
The new 100 percent soldier-baby society pretty rapidly runs out of locals, so they mount expeditions to break into other nests, bite the locals in half and steal their eggs. This is a species for whom murdering parents and enslaving the children is genetically programmed. It's a hive-minded hybrid of Hannibal Lecter and Hitler with a suicidally dedicated army of millions. We can therefore at least say this one isn't made by Satan, if only because he wouldn't risk creating his own replacement.