I can pretty much guarantee ya' some of the bullsnakes we have around here (5-6 footers) will have cats goin' vertical rather than horizontal! These things are masters of stealth! They aren't poisonous, but they'll hiss at a fella, or a critter of his, and scare the pants off about anyone including the furry critters and humans alike. And, they look so much like Rattlers, that they scare the be-jeezus outta' anything! And the hiss sounds just like a rattle, and they even shake their tail like a rattler (so fast you can't see it). They imitate a rattler perfectly, and apparently they've learned to do this over time...somehow.
edit - Bullsnakes are GREAT snakes to have around though, so never kill one! They're voracious eaters and eat all mice, rats and all sorts of other pests! I've got a neighbor who freaks out over them and panics. I go over to her place and grab the snakes and throw them in a bucket (and a snake can't get out of a bucket) , and cut them loose on our place!! The more the better!! Everyone should love bullsnakes! Seriously. They good citizens to have around. And, they don't eat the wrong stuff (well, they will snack on some chicken eggs now and then if you leave 'em around too long).
edit - Bullsnakes are GREAT snakes to have around though, so never kill one! They're voracious eaters and eat all mice, rats and all sorts of other pests! I've got a neighbor who freaks out over them and panics. I go over to her place and grab the snakes and throw them in a bucket (and a snake can't get out of a bucket) , and cut them loose on our place!! The more the better!! Everyone should love bullsnakes! Seriously. They good citizens to have around. And, they don't eat the wrong stuff (well, they will snack on some chicken eggs now and then if you leave 'em around too long).