Tough call. Militant abolitionists fighting to preserve Kansas as a Free State (notable as it's geographically isolated by Missouri, which was in the Union, but which allowed slavery). The Runnin' Rebels power comes primarily from their moustache and their revolvers. While they're not from Missouri, and Nevada was part of the Union, the "Rebels" bit implies Confederate sympathies. Which makes this not so much a mascot matchup, but more of exactly the plot of The Outlaw Josey Wales. Which means the Runnin' Rebel is Clint Eastwood. Without giving too much away in terms of spoilers of a 34 year old Western, let's just say that if Clint Eastwood movies have any connection to reality, UNLV advances.
#5 Michigan State Spartans vs. #13 Houston Cougars
Cougars are terrifying, but so are Spartans. And one of them has weaponry. I'm not sure if I've ever definitively decided that the mascot matchup is a one-on-one sort of deal or if several cougars are fighting several Spartans. Either way, I can't see how the cougars come out ahead in this. Even if you grant cougars the same equipment, cougars and weapons only ever win on heavy metal album covers. Michigan State advances
#11 San Diego State Aztecs vs. #14 Ohio Bobcats
This is starting to get a little specist, but historically, Aztecs and Bobcats may have interacted (though bobcats may not have lived far south enough). If they did, I'm pretty certain that the bobcats didn't come out victorious. San Diego State advances
#7 Oklahoma State Cowboys vs. #15 UC Santa Barbara Gauchos
An interesting matchup in what has quickly become the All-Human region, this looks like an interesting matchup on the face of it. A few things wind up influencing the outcome. First, look at the Gauchos logo. That, my friends, is a gaucho who knows a thing or two about steath. That's the ninja of gauchos. Additionally, this was evidently the Oklahoma State logo prior to 2000.
Is that Bruno Kirby? I think that's Bruno Kirby!
While that's a fine moustache, he's clearly going to have some manueverability issues. Not so with ninjagaucho. UC Santa Barbara advances