Carnivores eat small lizards, other dinosaurs, and fish. Some of the strongest creatures in the game are Carnivores. All aquatic creatures are Carnivores. Animals like Arizonasaurus can climb and are scavengers which can only kill very small babies, eat fish and lizards and steal from the true killers. The medium Carnivores like Allosaurus or Prognathodon can try attacking bigger animals and swim to lakes in order to ambush animals. Finnally, the apexes like Spinosaurus and Giganotosaurus are not a subject to mess with due to their damage, health and bleed. You should run away from these instantly if you can't fight well.
Herbivores eat ferns and trees. Trees are only consumable in spring and summer. Some Herbivores like Yandusaurus are easy prey though most of these prey items are quite speedy, while on the other hand you have the ones that a small or medium sized animal shouldn't mess with like Therizinosaurus, Futalognkosaurus or Styracosaurus. Towering over all of these by the means of damage, health and height are the sauropods. They shouldn't be messed with and you should stay away from them if you aren't expirienced in fighting.
Omnivores eat both meat and plants, meaning all food sources listed above are valid sources of food. However, most omnivores aren't able to reach tree leaves. Also, occasionally when an omnivore inspects a fern, it will provide the carnivore tooltip. Omnivores are the weakest of the three diet classes. The Gigantoraptor and Deinocheirus are both the most powerful of all the Omnivores. They are all bird-like non avian dinosaurs and are extremelly fast. Animals like Avimimus are fast and weak, making them scavengers and unable to fight. The Gallimimus is able to hunt instead of scavenge, making it a nightmare to babies. The gigantic Deinocheirus and Gigantoraptor are the only ones besides the Gallimimus able to defend themselves from large predators.
Diets are a way to classify dinosaurs, based on food source.
Diets can also change the play style for certain animals.
The Deinocheirus is the only Omnivore able to swim well.