The various dinosaurs and other creatures in can be converted into useful mounts for exploration and combat, but some are much harder to tame than others.
The size and aggressiveness of a creature are only two small factors that could determine how hard it is to pacify. Some dinosaurs take a much longer time to tame, requiring you to complete tasks or hunt down specific items.
Although most creatures can be tamed by knocking them out, you get better bonuses from beasts you tame when you do so non-violently.
This could include raising them from infancy or following them around and feeding them their favorite food until they are friendly with your character.
While many creatures tamed early in are easily won over without violence, later dinosaurs aren't so receptive. One of the more annoying creatures to tame is the Shadowmane, a cat-like creature that resembles a saber-tooth tiger with feathers.