In all fairness to dinosaurs, there is nothing to fear from them. They are all dead. Unless you count the birds they have evolved into, you simply aren't going to have a run-in with a T-Rex unless it's at a museum. That being said, there are hundreds of species of dinosaur that are absolutely terrifying in appearance—of which we'd be afraid to run into. Jagged rows of teeth, powerful armored tails, and their sheer size make a fear of dinosaurs a healthy proposition.
Some of the scariest dinosaurs are those that preferred to eat meat, so you won't find any herbivores on this list. That isn't to say a colossal animal who preferred to eat leaves wasn't something you should fear; a Brontosaurus could step on a person and probably not even know it. When it comes to fearing a scary dinosaur species, it's all about thinking one is chasing you.
Thanks to Michael Crichton and Steven Spielberg for making them realistic enough on screen to actually fear them, people who have watched any of the Jurassic Park films likely have an idea of which are the most dangerous dinosaur species to humans. Were we to travel back in time and stumble upon one, or if some careless scientist somehow figures out how to clone them (which seems plausible), perhaps it's wise to fear these colossal (and often ugly) monsters from the past.
Size: These were the largest of these types of dinosaurs having, achieved a size of up to 19 feet (8.7 meters) in length.
When They Lived: Early Cretaceous, 126 MYA.
Distinguishing Features: They were likely covered with a down-like feather with some larger plumage along their heads and appendages. Their most distinctive feature was their enlarged claw on each foot. These could reach a length of up to 9.4" (24 cm).
Why You Should Be Scared: If you watched Jurassic Park and insisted the creature called the Velociraptor didn't really live up to its ancient inspiration, that's because it mostly resembled the Utahraptor. They were likely very fast and may have hunted in packs, making them deadly to animals far larger than a single individual.
Size: 43 feet (13 meters) in length and weighing in at around 15.2 tons
When They Lived: Late Cretaceous, 98 to 97 MYA
Distinguishing Features: When it comes to a distinguishing feature, the Gigantosaurus' size is what separates it from most other theropod dinosaurs.
Why You Should Be Scared: Gigantosaurus was one of the largest known terrestrial carnivores. Were you to somehow wake up in the late Cretaceous period and stumble upon one, it might decide to swoop down and bite you in half, which it could easily do. Some specimens have indicated they could grow much larger than the infamous T-Rex, but their size alone wasn't their biggest concern to its prey; it was fast. It is believed to have been able to run up to 31 mph (50 km/h) making it a tough animal to run away from... for very long.
Size: Around 40 feet (12.3 meters) in length weighing in ar around 9-10 tons.
When They Lived: Late Cretaceous, 68 - 66 MYA.
Distinguishing Features: Their size, jaws, and funny little arms are their most recognizable features. Most people know of their terrifying teeth, but few realize they were probably covered with feathers.
Why You Should Be Scared: The T-Rex is probably the most infamous of all theropod dinosaurs whose suffix literally translates into "King". Thanks to a number of films to include the entries in the Jurassic Park franchise, many people have a healthy fear of these large carnivores. T-Rex's were likely very fast for their size and were very strong and powerful animals.
Size: 41 feet (12.6 meters) in length with an estimated weight of around 3.1 tons
When They Lived: Cenomanian-Turonian ~97 to 93.5 MYA
Distinguishing Features: Mapusaurus shares many similar features with other theropods. They have small, vestigial forelimbs with strong and lean hind legs.
Why You Should Be Scared: Their heads are large enough to swallow any lost time-traveler whole should they decide to give one a try. The Mapusaurus was certainly strong and capable for its size, but you would only ever run into one if you were visiting a prehistoric Argentina and possibly Chile.