Hexapody? I tried looking up this term on Google, but I can't find it...
Well, let's list the prehistoric animals in the Monster Manual, Monster Manual II, and Fiend Folio (actually, none in the latter I can find) then...
Axe Beak
Baluchitherium
Bear, Cave
Dinosaur, Anatosaurus (Trachodon)
Dinosaur, Ankylosaurus
Dinosaur, Antrodemus (Allosaurus)
Dinosaur, Apatosaurus (Brontosaurus)
Dinosaur, Archelon Ischyryas
Dinosaur, Brachiosaurus
Dinosaur, Camarasaurus
Dinosaur, Ceratosaurus
Dinosaur, Cetiosaurus
Dinosaur, Dinichtys
Dinosaur, Diplodocus
Dinosaur, Elasmosaurus
Dinosaur, Gorgosaurus
Dinosaur, Iguanodon
Dinosaur, Lambeosaurus
Dinosaur, Megalosaurus
Dinosaur, Monoclonius
Dinosaur, Mosasaurus
Dinosaur, Paleoscincus
Dinosaur, Pentaceratops
Dinosaur, Plateosaurus
Dinosaur, Plesiosaurus
Dinosaur, Pteranodon
Dinosaur, Stegosaurus
Dinosaur, Styracosaurus
Dinosaur, Teratosaurus
Dinosaur, Triceratops
Dinosaur, Tyrannosaurus Rex
Hyena, Giant (Hyaenodon)
Irish Deer
Mammoth
Mastodon
Rhinoceros, Woolly
Shark, Giant (Megalodon)
Tiger, Sabre-Tooth (Smilodon)
Titanothere
Wolf, Dire
Dinosaur, Ankisaurus
Dinosaur, Camptosaurus
Dinosaur, Compsognathus
Dinosaur, Dacentrurus
Dinosaur, Deinonychus
Dinosaur, Dilophosaurus
Dinosaur, Dimetrodon
Dinosaur, Euparkeria
Dinosaur, Kentrosaurus
Dinosaur, Mamenchisaurus
Dinosaur, Massopondylus
Dinosaur, Nothosaurus
Dinosaur, Ornitholestes
Dinosaur, Phororhacos
Dinosaur, Podokesaurus
Dinosaur, Pterosaur, Giant
Dinosaur, Struthiomimus
Dinosaur, Tanystropheus
Dinosaur, Tennodontosaurus
Hmm - unfortunately, I don't see any glyptodonts, giant sloths, or carnivorous kangaroos here, not to mention any marine arthropods...
Perhaps we can take any of the various "giant" creatures (giant crayfish, giant marine spider, etc.), though, and find the most similar prehistoric actual variant out there...
The dinosaur lists are quite extensive (for a monster manual), but not everything in there is actually dinosaur
Do you see any therapsids or pelycosaurs in there? (I defer to your far greater knowledge in this area...
)