3Megalania prisca, the Giant Lizard
Komodo dragons of today are not to be fucked with, reaching 10 feet in length and weighing up to 150 pounds. Also, they're quite poisonous and can very much kill you. Now take one of those things and double its size. Then make it anywhere from five to 30 times as heavy. That's the Megalania, aka the giant ripper lizard. And us modern-day humans just missed them.
What a shame; they looked playful.
As recently as 40,000 years ago, Megalania patrolled the Australian Outback, living up to its "ripper" name with little difficulty. And while anyone who knows their history laughs at the idea of humans and dinosaurs walking the Earth at the same time, your Australian ancestors experienced something pretty damned close -- it's likely the earliest human settlers in the area ran into these monsters, and then promptly ran away. As well they should have.
And unlike today's Komodo dragons, whose saliva may or may not be somewhat poisonous, Megalania were absolutely venomous, and it was potent as hell. If the soft marsupials of the time ever questioned who ruled the roost, Megalania and its toxic spit was more than happy to remind them. Again and again, if need be.
Via Wikimedia Commons
And, as shown here, they could live for tens of thousands of years without skin or internal organs.
Now remember: All of this occurred 40,000 measly years ago. As far as history is concerned, we crashed the party a mere millisecond after these gigantic fuck-yous from Mother Nature stepped out for a smoke. They were here so recently, in fact, that some people think they still exist. Those people better hope to God that they're wrong. With the seemingly infinite number of murderous creatures that Australia currently sports, do we really need a real-life poisonous Smaug making some grand return to ensure our final days on Earth are completely unpleasant?
2Basilosaurus, the Giant Whale
Via Nobu Tamura
OK, yes -- today's whales are plenty huge. We're aware of this; we're also aware that they used to be a lot bigger. And meaner.
Via Wikimedia Commons
That's the Basilosaurus, otherwise known as the Joker's favorite fish. At over 80 feet long, it dwarfed the killer whales of today, which only average 25 feet or so in length. So yeah, you'd need three Shamus, plus your terrified, piss-stained ass, to equal the Basilosaurus in stature.
When its remains were discovered in Louisiana in 1832, it was mistakenly identified as a sea monster, and given a name that literally translates to "king of the reptiles." They forgot to tell people that this thing went extinct quite a while ago, and so local fishermen shat themselves over the thought of a gigantic monster lurking in the same waters they used to find ingredients for catfish gumbo.
They wanted 25 cents to see that thing? What a ripoff. Wasn't that like the price of a house back then?
Luckily for them, Basilosaurus doesn't exist anymore; if it did, the fishermen would be the gumbo. That jawline would make Basilosaurus a top predator in any time period. Set the time machine forward 20 million years, and it would be duking it out with Livyatan melvillei and Megalodon, no slouches themselves. Another 15 million years, and it would be chewing up Jaws like Jaws would chew up a mackerel.
Even today, it would take one look at the 100-foot, 170-ton blue whale, realize it's just a big teddy bear unless you're, say, a piece of plankton, and swim off all smug-like, knowing the delicious dishes of the sea are still available, and still all for him.
"Mmmm. After-dinner mint."