The boars are protector and inhabitants of the Cedar Forest. The Moro clan is their ally, but they have different goal regards fighting Irontown. The boars want to kill all humans, including Moro's daugther San.
Humans are their archenemy, because thehumans destroy their habitantfor resources. Nago's army protect the forest against humans many times. But one day he get killed by Lady Eboshi with cannons. Later on Nago's mind get polluted makes him a demon.
Through the long history of fighting humans, the boars become weaker. Their piglets are small and weak. This issue cause that there will be no more boar gods in the future. The boars don't stand a chance against humans, because of the superior number of humans alone.
The boars are proud being pigs with tusks. They can't cope with being beaten by humans, a small species (without tusks). In combat they know neihter loss nor fallback. They take any challenge, even though it is a trap setting by humans. Okkoto explains that the boars don't want to get forgotten. So the humans won't forget the proud boars.
In the movie three boar leaders are seen. They are bigger than their kind. No boars object the opinion of their leaders. Okkoto has the ability to see the past by sniffing someone's arm. He sniffs Ashitaka's arm to see Nago's death. The humans fear their leaders, because of their size. They also get called gods by humans.