Everything is sized incorrectly. The "secret" animal is said to be 32-45ft tall. In comparison, that would make the hunter well over 20ft!
Also, Archeoptryx, the main aerial animal, is not actually from the Ice Age. This also goes for Diatryma, the carnivorous bird, and Brontothiery the rhino like animal with the "Y" shaped horn. Brontothiery and Archeoptryx are also spelled wrong in the game, Brontothiery actually being Brontotherium.