The boars are protectors and inhabitants of the Cedar Forest. They are allies with Moro's clan, but they have different ideas regarding fighting Irontown. The boars want to kill all humans, including Moro's daughter San.
Humans are their archenemies, because the humans destroy their habitat for resources. Nago's army protects the forest against humans many times. However, one day Nago was killed by Lady Eboshi with cannons. Later on Nago's mind got polluted and he turned into a demon.
Through the long history of fighting humans, the boars became weaker. Their piglets became smaller and weaker as their numbers dwindled. This issue caused that there would be no more boar gods in the future. The boars didn'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 neither loss nor fallback. They take any challenge, even when it is a trap set 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 the boars' leaders, because of their size. They are also called gods by humans.