The Apatosaurus is just a recycled animation model of Diplodocus, without back-spikes and with a different coloration. Since they were very closely related, the animators didn't need to build a completely new model for Apatosaurus. Similarly, the Othnielia and Dryosaurus are slightly tweaked and recolored variations of the original hypsilophodont model.
The small herbivorous dinosaur that Al tries to catch, Othnielia, has been renamed Othnielosaurus since this special aired, since many fossils originally thought to have belonged to Othnielia had proved to be those of a different dinosaur. Othnielia is considered by some palaeontologists to be a dubious name.