It was a Tyrannosaurus - legend has it that Disney himself decreed that the dinosaur be given three fingers because "it looked better". At the time the forelimbs of T. rex hadn't been found, but it was given two fingers based on related genera (like Albertosaurus).
That painting inspired KingKong's Rex (also 3 fingered) and the Fantasia.... And Gwangi later!! (wich was also a Rex based on Knight's painting and Kong's, though later Harryhausen said it was an allosaurus trying to pacify the angry purists).
PD: in the JP novel, the young Rex attacks the stegosaurus at the end. ;D A "Fantasia" nostalgic reference, I suppose.