Smilodon for the camera!
I could have picked an image with just the cat, but when in doubt, use Frazetta!
Continuing in the lost world theme, we have the sabre tooth tiger. Our main predatory mammal for such areas. Of course, such a creature would have had a hard time competing against the likes of T-Rex, but during the ice age, when dinos were long gone, ol' Smiley did all right for himself.
I've already done a Random Monster Assessment on tigers, and really smilodon is more of the same. It's a great big cat with teeth and claws. Granted, in BX, it's the biggest and toothiest of the bunch.
Sabre-tooth Tiger (from Moldvay)
Move: 150' (50')
Att: 3 (2 claws/1 bite)
No. App: 1d4 (1d4)
The main thing I would note about this fella is that he is (was) an apex predator and not easily scared off (high morale). While -in real life- primitive man and the great cat would probably have done their best to avoid each other (top predators like their space), and the beast would have likely found easier pickings than the hairless apes with pointy sticks, competition over food might cause clashes. Maybe a clan of cavemen manage to down a mastodon. It would take more than a day to butcher such a kill and prep it for taking back to the cave. A pack of sabre-tooths might find the carcass in the night and decide to try staking a claim.
A party of PCs from "normal" times that find themselves in a lost world would probably be a smaller group than the smilodon typically would have to deal with (neanderthal mammoth hunting parties were typically a dozen or more men) . Snagglepuss might decide a handful of people are easy enough prey (or their horses). Especially if it's a pack of 3 or 4 cats.