“Brontosaurus” is usually cited as being originally reconstructed with elements from Camarasaurus, though some Brachiosaurus material may have also found its way in there.
Interestingly, that’s not why the name “Brontosaurus” was dropped from scientific usage. Even without the added material, Brontosaurus excelsus was considered a valid species, but in 1903, someone argued that it was similar enough to Apatosaurus ajax to warrant being moved into the same genus. Because Apatosaurus had been named two years earlier than Brontosaurus, its name took precedent, leaving us with Apatosaurus excelsus.