The Brontosaurus (now called Apatosaurus) certainly fits the description 'colossal', being the biggest animal that walked the earth, and seems to match the footprints.
But how does it match the hint 'prey'? The Brontosaurus was a plant-eater.
Because of his size, he was not a likely prey. The biggest meat-eater living in the same area at the same time was much smaller and would be crushed by Bronto's defensive tail. T-Rex did not come into existence until millions of years later.
Of course in Jurrassic Park anything is possible, so from the movies point of view the T-Rex can be seen as a likely enemy with Brontosaurus as it's prey.
Exactly in Jurrassic Park it would be a prey, if thats the case you got to give it up to Sideshow for going such distances to try and stump us, using foot prints and such.