All prehistoric creatures behave differently, and therefore each species has a different level of danger. You must become a better trainer by reaching higher Player Levels in order to care for more dangerous creatures.

If a creature is too dangerous for you, there will be an icon on their card in the Enclosure view and the placement pop-up. You will not be able to place this creature or spend care items on it until you reach a Player Level the DPG deems ready.

Most creatures of each rarity share similar danger levels, but there are exceptions. Some creatures, such as the T. rex, are just very aggressive by nature, and take more experienced players to care for them.