What animal frightens you most, and why?
Some nonhuman animals are dangerous...some are annoying or look scary...but humans are the ones with the real power and unpredictable malice. Very few animal predators look on humans as possible prey (a few do--I don't swim with sharks, or stroll up to polar bears to pet their fur), but human predators readily regard humans as prey. And they're smart, sneaky, and often have deadly weapons, not to mention opposable thumbs.
If you insist on the choice of another species...dogs. Dogs kill and injure people more often than venomous snakes, in this country. Dogs owned by irresponsible, criminal, or vicious people--loose dogs, especially in packs--are a constant danger. Mind you, I like dogs...if they are properly trained and under control. The problem with dogs is, once again, homo sapiens...humans.