When asked to name some dinosaur names, a few names come to mind: T-Rex, Stegosaurus, Brontosaurus. Unfortunately, there's no such thing as a Brontosaurus.
The first "Brontosaurus" skeleton was discovered by paleontologist O.C. Marsh in 1879. Since the skeleton he found didn't have a head, he used a skull that was found miles away to complete the skeleton. He named the hybrid-dino-skeleton "Brontosaurus" (Thunder Lizard).
It turns out that the skeleton that Marsh found was actually the skeleton of an Apatosaurus, a species that Marsh had already discovered and named two years earlier. Since the dinosaur in question already had a name, there was no longer any dinosaur named Brontosaurus.
The supposed head of the Brontosaurus was later discovered to be from a different dinosaur known as the Camarasaurus. This means that not only is there no such thing as a Brontosaurus, but also there is no dinosaur that looks like what the Brontosaurus looks like. It was literally the head of one dinosaur on the body of another dinosaur.
(Sources: 1, 2)