Until now the Triceratops and the lesser-known Torosaurus were the thought to be the same type of dinosaur.
The Torosaurus with its big horns was meant to be grown up and the Triceratops was meant to be a youngster, with smaller horns.
Now researchers at Yale University in the US who've been looking at dinosaur fossils say they are actually two completely different species.
They looked at 35 examples of each dinosaur and found that Torosaurus skulls had longer frills at the back with large holes while Triceratops has a smaller, solid frill.
Despite their fearsome horns, both the Triceratops and Torosaurus were plant eating dinosaurs.