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.