Brontosaurus is the beloved old museum name that refused to die. It was a large diplodocid sauropod from the Late Jurassic of western North America, with the same general long-necked, long-tailed body plan people associate with the great sauropod halls of childhood. The modern case for bringing the name back is not based on nostalgia, but on anatomy: some researchers argue it was distinct enough from Apatosaurus in its skeleton to count as its own genus.
That said, Brontosaurus is still best understood as very close to Apatosaurus, not as some wildly different beast. Think of it less as “the fake dinosaur” and more as “the dinosaur whose name got demoted, then later argued back into good standing.” That is why the name feels so emotionally familiar to people who grew up with it, while still carrying a bit of scientific controversy.