Kaprosuchus: ok, ok. I know I have described some of the other reptiles as terrifying, but I think this one is especially insane. Take a crocodile. Turn 3 pairs of its teeth into long fang/tusks. Give it some horns on the back of the head. Move the eyes forward a bit so that it can better focus on its prey and have depth perception. Finally, take it out of the water and straighten its legs so that it can run properly. Honestly all you would need to do is give it fire breath and some wings and it would be a dragon.