It tests whether your mouse movement looks human–we’re really bad at things like moving in straight lines, so it’s pretty evident from a mouse movement log whether you’re a human or a simple bot. It also takes a bunch of auxiliary browser/environment data into account. It’s not perfect, but it’s complicated enough to defeat to provide fine protection against cheap spam.
Probably Hercules the Liger. Terrifyingly enormous animal–pictures do not do justice to how intimidating a predator of that mass is.