Apatosaurus lived during the Late Jurassic of North America, 155-145 Million Years Ago. It belongs to the Diplodocidae Sauropods, obviously a relative of Diplodocus. Apatosaurus, means Deceptive Lizard, in Latin/Greek. It was a 30 Metre Long, bulky 35 tonne Sauropod; a 'fortress of flesh'. Only the young, injured, sick or old were at risk of predation. On the other hand, a healthy old individual could be virtually untouchable beyond a certain age, for sheer size alone. A lone Allosaurus, or even a pack, could stand little chance of taking on a fully grown and mature Apatosaurus, hence why they would likely target smaller, weaker and generally more vulnerable animals.
Apatosaurus only appeared in the Walking With Dinosaurs Special - The Ballad of Big Al. In this, it receives two short scenes. Firstly, it is shown in the explanation of the various kinds of prey in Al's Late Jurassic World, seen moving across screen as a herd of Sap Green coloured (with a dappled pattern of lighter greens over this) Sauropods, descrived as being too large to attack, even for fully grown adult Allosaurus. The second and last time we see them is when a small herd of them are seen moving through deserts in a brief scene showing the plight of Big Al after his fatal fall.
However, it could also be said that we also see the Apatosaurus before and after the story of Big Al plays out - as a skeleton in the Wyoming Geological Museum during the Intro and the lead to the end credits.