In Animal Farm, it quickly becomes clear that Napoleon, a "fierce-looking" boar, is an opportunist and a tyrant, and we see this is a number of ways.

Firstly, in Chapter Two, Napoleon steals the cow's milk and gives it to the pigs to be mixed in with their mash, instead of sharing it with the other animals. Napoleon does not explain this decision to the others. Instead, he gets Squealer to tell the other animals that the pigs need the milk more than the others because it is scientifically proven to help brain function. The animals accept this explanation but it demonstrates Napoleon's selfish and opportunistic nature. 

Secondly, in Chapter Three, Napoleon takes Jessie and Bluebell's nine puppies and raises them himself in a loft. He claims that he is educating the puppies but, in Chapter Five, the animals learn his true intentions. During the disagreements over the windmill, Napoleon sets the puppies, which have now grown into fierce guard dogs, on Snowball. The other animals are "silent and terrified" by Napoleon and his dogs. The animals notice, however, that the dogs wag their tails for Napoleon, just like they did for Mr Jones. This is an important observation: it shows that Napoleon is becoming a tyrant and will use force to assert his will over the others. 

Thirdly, another of Napoleon's key actions is to change the seven commandments to a single rule: "All animals are equal but some animals are more equal than others." This is representative of Napoleon's near-transformation into Mr Jones; he carries a whip, walks on two legs and drinks alcohol. He has become the master that the animals sought to eradicate. This coincides with Napoleon's renaming of "Animal Farm" to "The Manor Farm."  The animals are "half-frightened" to even go near the farm house and, when they do, they find a scene in which it is possible to distinguish pigs from humans. 

In many respects, the story of Animal Farm has come full-circle. In Chapter One, the animals sought to rise up and overrule Mr Jones and, by the end, Napoleon has become the new tyrant.