Apply one or more of the following tags to the statblock of familiar modern specimens, from aardvark to zebra.
Cave: +2 HD, all damage dice raised to the next increment
Dire: +2 HD, +2 damage per attack, +2 Morale
Giant: x2 HD, x2 damage, Armour Class as Chain.
Horned*: +2 HD, gives a charge attack, knockdown on a special.
Sabre-toothed*: +2 HD, bite damage raised to 2 dice or next increment if already multiple
Woolly: +2-7 HD (d6+1), +4 damage to main attack, +2 damage to other attacks
* and/or Tusked.
HD increases stack. Dice increments stack. Plusses to damage, take the highest.
Damage multiplier applies to dice only, not adds.
Apply HD multiplier to the base statblock or after applying any other tags, or wherever it falls in your animal's descriptive title.
If HD rise to 8 or above, you can instead handle the prehistoric mammal as if it was an 8 HD dinosaur with 3d6 attacks. Such animals are between the size of a modern rhino and an elephant, and save at +4 vs. spells that don't inflict damage and vs. poison/venom.
If HD is approaching, reaches or exceeds 16, then you could treat the mammal as a 16 HD dinosaur with 6d6 attacks. They're as big as Dinosaurs (of course), and immune to spells that don't cause points of damage and poison/venom.
Armour Class as the original, or equal to Unarmoured or Leather. Mega-armadillos and mega-porcupines etc can have Scale to Plate protection.
Prehistoric mammals live in arctic and subarctic conditions, are immune to normal cold, and save at +1 and take -1 damage per die vs. magical cold. Because it's the Ice Age. Always. Unless it's a steamy jungle-type Lost World Beyond the Ice.
They must make a Morale Check if you use fire or firearms against them.
Specials (19-20 vs. human sized etc) will be bear hugs, tramples or rending/auto-bite. Swallowing you whole can be a possibility, but doesn't seem as thematically strong for mammals as it does for dinos.
Walruses, weird whales, ice-breaking horned manatees and long-necked seals can sink boats and/or snatch folk from the deck. Porcupines can fling a volley of spikes like a Manticore. Spiked and clubbed tails are in the mix.
The idea of Horned Sabre-toothed Giant Woolly Vampire Cave Bats is of course ridiculous.
All herbivores are delicious and nutritious and go a long way.
Carnivores have at least one organ of such concentrated nutrition that it will kill a modern human(oid), but is a delicacy to Cave Men.
Cave Men are equal to Bugbears, and have at least 16 Strength, immunity to normal cold and bonus/resistance as other prehistoric mammals.
The language barrier between them and modern humans cannot be overcome by either comprehend languages or speak with animals.
If they don't think you're gods, they might think you're food, and they possess a plentiful resource unrecognised by them as useful or valuable to modern people.
Flavour according to your setting and the last thing you read/saw about prehistoric human society.
Since 2003, you're allowed to include Cave Hobbits. Either 1+1 HD and Basic Halfling abilities or use Athasian Halflings.
Neanderthals are Neanderthals, RAW or seasoned as you like.
