You happen to have hit on one of my passionate hobbies. Although it's a bit of a tangent, let me address each of these in turn.
1) Smilodons are very cool, but not any bigger that tigers or lions (maybe a hair smaller).
That's true for the north american species,
Smilodon fatalis, although that would have been a
big lion or tiger. The south american species,
Smilodon populator, is however one of the largest species of of the cat-family known, period, and would have been quite a bit bigger than even the largest lion or tiger today.
Although keep in mind, that both the European cave lion,
Panthera leo fossilus and the American lion,
Panthera leo atrox, would each have been a good 25% larger than the largest population of African lions today, too.
radferth said:
2) Mammoths are not that much bulkier than African elephants, although they are much taller.
No, they weren't. In fact,
wooly mammoths would have been a bit smaller both in height and in weight. However, there
is one species of mammoth.
Mammuthus sungari, which is quite a bit larger than most elephant species today.
However, the mammoths of North America and the wooly mammoths (of north america, Europe and Siberia) were not. It's also interesting to note that mammoths are actually true elephants, and are in fact more closely related to Asian elephants than Asian elephants are to African elephants. If the Columbian mammoth had survived a little bit longer, to have been discovered by Europeans, then it would almost certainly merely have been called the American elephant.
radferth said:
3) A lot of extict ice-age animals actually survived quite a ways into our current thaw. Mammoths were still alive on some islands in the north Atlantic a few thousand years ago. (Can't remember if it was 4000 years ago, or 4000 B.C.).
That is true; and it's also difficult to prove that something
didn't survive longer just because you don't have record of it. You can't prove a negative. Depending on how generous you are with accepting circumstantial evidence, there's a lot of it kicking around to suggest that many Pleistocene mammals actually survived into the historical period and we literally just missed seeing them.
radferth said:
4) Australia had 20' monitor lizards as recently as 20,000 years ago. Think of a double-sized Komodo dragon.
Now they're saying 40,000 years ago instead. Still, though... cool.
radferth said:
5) Hyenadons were very early mammals that were a bit like a hyena. In the much more recent past there were extra-large hyenas that weren't called dire hyenas, but it's probably a better name for them than hyenadon.
Actually, they weren't really very much like hyenas.
Hyenadon means hyena-tooth, but otherwise their anatomy was pretty different.
radferth said:
6) Cape buffalo aren't extinct, but would be pretty good monsters to fight. Has anyone seen stats for those?
I haven't. Then again, all large, aggressive wild cattle-like creatures can probably be pretty well represented by bison stats.
radferth said:
Folks have mentioned Irish deer, cave bear, short-faced bear, smilodons, and aurochs as the actual names for some big, extinct mammals. I'd like to add spotted lion and titanothere as the real names (IIRC) for big, old lions and big, old rhinos. Not sure how close they are to modern rhinos, but I think they are what the LotR movie-folks based the Grond-pulling beasts off of. And I don't know how they know the lions were spotted. Cave drawings, maybe?
Spotted lions are cryptids, actually. Nobody's ever confirmed that they really ever existed. You probably mean the above-mentioned 25% larger European cave lions or American lions.
Titanotheres weren't really closely related to rhinos at all, and in fact were only superficially rhino-like horse relatives, of all things. Their "horns" couldn't be used the same way as a rhino's horns, as they were oriented side to side rather than front to back, plus they were rather slender and made of bone rather than keratin. It's not actually clear what they did with them. They may have been more for display than anything else.
In any case, titanotheres are also just another name for brontotheres, and I know stats for them have popped up somewhere in the various monster books before, and could be easily adapted to Pathfinder.