Polar bear.
Big assed carnivore, and easy to see in the dark, too.
Tris
Okay then, I choose Darth Vader as my Patronus, Vader from the OT, not the wussified Anikan Vader from the last prequel
Now that I think about it, probably an owl.
Moose. They’re just such big, gangly, goofy animals and definitely Typo’s and my favorite non-pet critter. We have a number of stuffed mooses (moosi? meese?) around the house.
Cthulhu.
Oh wait, that’s HP…Lovecraft.
And when I think about it, he might tend to ruin the entire “think happy thoughts” part of the Patronus. You know, summon your Patronus, be terrified, lose the Patronus…
Mine would be a Stobor.
So, do evil wizards have a patronus? Maybe their happy thoughts are torturing their victims and their patroni reflect it?
In other words, it wouldn’t exist?
A shark with a frickin’ laser beam on its head.
Mine would be a grizzly. My name is Bernard, meaning “bold as a bear”.
Either that or Trogdor the Burninator.
According to Rowling, no, Death Eaters, with the exception of Snape, do not have Patronuses. Not because they can’t, but because they don’t need them:
It seems that other “nasty” wizards can use the Patronus, as we see when Dolores Umbridge summons her Patronus to stand guard against the Dementors at the Mudblood trials. She’s not a Death Eater, however.
So I guess it depends on how you define “evil”. Rowling separated the ranks at Death Eater or not. But really in the end, what does she know about it? She’s only the author. 
To match my already chosen theme song to the comedy that is my life, it would have to be a baby elephant.
Somebody beat me to the Tasmanian devil, so I’d guess I’d go American with a wolverine. Small-w type, not stocky Canadian guy type.
The only pets I’ve owned are turtles and catfish. Maybe one of those giant Southern catfish that could whip Dementors with their whiskers. As for turtles, maybe snapping or giant sea turtles would be cool. My 3-inch painted ones from the 70s, nah–the only thing they could do was maybe give you salmonella, which is why they were banned.
You think the elephant would be the only one startled?
What do you mean? You built all these traps for them, didn’t you?
I suspect my totem animal is a hedgehog.
I tried explaining this to a friend who asked if Umbridge was good or bad when she was part of the way through OOTP. She was looking for a simple answer, but when you think about it, it’s hard to class her as one or the other.
I think Umbridge is pretty much evil, or at least cruel and heartless. She can be evil and nevertheless oppose Voldemort. Stalin fought Hitler, after all.
Didn’t Rowling actually say something to this effect? “The world isn’t divided into good people and death eaters”, or something like that? It might have been in reference to Snape rather than Umbridge, though.
With regard to the original question, after intense soul searching I’m leaning heavily towards some sort of flying hippopotamus.
T-Rex
T-100
Pretty much anything that begins with “T-”