OK, here are all the "age of mammals" critters i have stats for. let's narrow this list down. a lot of these animals are very similar - we don't need 5 types of bears and rhinos.
Monster Manual:
Hyaenodon - a huge canine that resembles a hyena. (5 HD)
Irish Deer - as big as a moose with enormous antlers (4 HD)
Lion, Spotted (aka cave lion) - large, fierce lions (6 HD)
Titanothere - huge, fearless herbivores that roam temperate plains in herds. Resembles a rhino with a flat shovel-like horn, charges and tramples to attack. (12 HD)
Monster Manual II:
Phororhacos - large flightless bird with a huge head and powerful beak (7 HD)
Dragon 137:
Alticamelus - relative of the camel, with a long giraffe-like neck. can bite or trample. (5 HD)
Ambelodon - a type of mastodon with shovel-like tusks, unsuitable for combat. Attacks with two stamps. Lives in swamps and rivers where vegetation is abundant. (9 HD)
Astrapotherium - rhino-sized herbivore with two dagger-like canines. Lives near water like a hippo. (3 HD)
Bear, Short-Faced (Arctodus simus) - stands twice as tall as a grizzly, due to long legs, and has a short muzzle. Continues to fight down to -9 hp (like a bear). Roams sub-glacial plains. Attacks with bite, and 2 claws can hug. (7 HD)
Bison, Giant - live in large herds in plains, and have a horn spread of 7-10 feet. Can attack with 2 horns, or trampling charge. (6 HD)
Cattle (Auroch) - wild cattle that are slightly larger and tougher than normal animals, 7' high at the shoulder. Also more fierce and agile than normal cattle. Males have 2 horns attacks and can trample on a charge. (6 HD)
Deinotherium - a distant relative of the elephant with a small proboscis and short tusks that curve downward. More like an Oliphant than a mastodon. Attacks with two tusks and 2 stamps. (10 HD)
Gigantopithecus - a huge ape taller than a gorilla that roams forests in small family bands. Attacks with bite and 2 slams, with which it can rend. (6 HD)
Glyptodon - 9-foot long ground sloth with an armadillo-like shelled appearance and a long spiked tail with which it attacks. (7 HD)
Macrauchenia - looks like a camel with an elephant's trunk. Attacks with 2 forefeet.
Moropus - looks like a horse with sort ears, but has short heavy claws with which it could dig and pull down tree limbs to feed. Could also defend itself with the claws and bite. (4 HD)
Sivatherium - relative of the giraffe that looks like a huge moose. Has two short, bony horns and two huge antlers. Can head-butt or attack with 2 hooves. (4 HD)
Sloth, Ground (essentially Megatherium)
Uintatherium - is the size of a large rhino; herbivorous but has large molar fangs. It can attack with these, or trample on a charge.
Dragon 167:
Agriotherium (savannah bear) - huge, primitive bear (slightly smaller than cave bear) with a dog-like head that roams the plains. Same attack routine as other bears. Fights until -9 hp like the short-faced bear. (8 HD)
Amphicyon (bear dog) - looks like a bear with a wolfish muzzle and canine tail. Claws and bites, but has no hug. Solitary, and attacks creatures larger than itself. (7 HD)
Anancus (pike-tusked elephant) - an elephant adapted to temperate woodlands. Has two 10-foot long tusks that it can attack with, and 2 stamps. (10 HD)
Andrewsarchus (giant long-jawed hyena) - the largest known carnivorous land mammal. Looks vaguely like a hyena with a 3-foot long skull, and a feline tail. A scavenger, but can attack with bite. (9 HD)
Camel, giant - hump-less camel that grazes on the plains. Attacks with a bite or two hooves. (7 HD)
Elephant, dwarf - island dwelling elephant that is only 3 feet tall. Attacks with tusks. (3 HD)
Hippopotamus, giant - massive hippo with huge tusks and periscoping eyes. (10 HD)
Megalania (giant plains lizard) - a giant plains lizard that resembles monitor lizards and komodo dragons, with long serrated teeth. Can knock down foes with a tail attack. (7 HD)
Metridiochoerus (giant warthog) - swine that fights with two sets of tusks. (5 HD)
Pelorovis (long-horned buffalo) - a relative of the buffalo with huge 12-foot long horns that curve out and down. It can charge to trample and knock down. (6 HD)
Sarkastodon (long-tailed bear) - looks like a bear with a long tail, fights with claws and bite. (9 HD)