Wooly mammoth or mastodon.
Paraceratherium (largest mammal ever)
Short faced bear - giant genus of extinct North American bear that estimates suggest weighed in between 2500 and 4000 pounds. Imagine a polar bear or a Kodiak bear TIMES TWO. Following a wounded one into a cave would require a backup gun…and a backup pair of shorts!!!!
Tyrannosaurus Rex (and other large therapods) - bonus points if you get within 50 yards and take it with a single shot firearm or heavy crossbow
- the 8 ton beast wouldn't be half as heavy to transport as the shooter's massive balls (provided they weren't being digested by a very cranky and still alive t-Rex)
Argentinasaurus or Patagotitan or any of the contenders for largest terrestrial creature ever, the big sauropods or titanosaurs from South America. I think for those you'd have to get underneath it and shoot straight up through the bottom of its head to hit the brain, then run like hell so the falling beast didn't crush you to death. A 60-100 ton 20-40 foot high 80-120 foot long creature crashing to the earth would be dangerous indeed.
Triceratops….definitely triceratops…everything about that dinosaur screams "come at me bro"
…or maybe "do you feel lucky, punk?" The Cape buffalo of the Cretaceous.
And a really cool contender: Quetzalcoatlus or Hatzegopteryx, the largest creatures known to have ever flown. 30-40 foot wingspan, carnivorous, weight very hard to estimate but not less than 300 and not more than 800 pounds…a flying grizzly bear to put it another way…closest thing to a dragon this earth has seen. A great big shotgun for those guys, like a 6 or 4 guage? Image the commotion of such a creature falling and flailing to the ground from the sky…
For everything else, my 450'ackley for guns I actually have, perhaps a bazooka or a 20mm auto cannon if anything goes..