So, I'm watching my man Jeff Corwin, when he shows me the stuff of nightmares: the coconut crab! [transcript of the episode] Jeff says they can grow up to three feet in length (head to tail) and weigh forty pounds. He'd picked one up and it was friggin huge! It was the size of a small-to-medium dog, omnivorous (including carrion) and the really scary part is that they climb trees. It's only a small evolutionary step to learn to jump down on passing victims. Being hit on the noggin with a forty pound crab would probably knock a guy out, leaving the crab to feast at it's leisure. Or, if they learn to work together, they could rip you into bite-size crab snacks.
Most of the information I can find on them refers to crabs from Oceania (and it talks about them as food), but the one Jeff found was on Zanzibar in East Africa.
For those into that type of thing, some boobies to spice things up.
If you're into Jeff, this ep ("Zanzibar: Dr. Corwin I Presume") has sexy-shirtless Jeff.