On the 2011 bibliography from my Sakura-con Spirited Away panel there is a website created by Professor Susan Blakeley Klein at UC Irvine for her Japanese ghosts course that mentions mononoke. Here it is defined as, "A general term for possessing tama, often female; can be living/ikiryô or dead/shiryô." It was asked of me whether this, "Was the intended "background" viewers were supposed to have of San in Mononoke Hime? Was it Studio Ghibli's intention for her to be "possessed" by the forest gods and eventually freed by Ashitaka?"
There's a fascinating online article by professor Tanaka Takako about the origin and changing meaning of mononoke from its origination in the Heian era up to its present meaning. Click here to read it in the archives of the sadly no longer publishing Japan Echo. As Klein puts forward in her table, mononoke are mysterious formless beings that take possession of a human host in order to actualize whatever it is that they want, be it revenge or other.
The female element Klein mentions comes from one of the most classic examples of spirit possession in Japanese literature: Genji Monogatari. In the story Genji's wife, Lady Aoi, is possessed by the spirit of Lady Roku, who was the mistress of Genji for some time. She died but her jealous is so strong that her spirit lingers in this world and haunts their residence and Genji's wife. Lady Aoi is pregnant, which makes her even more susceptible to the malicious effects of spirit possession. Pregnancy automatically links the mother back to the spirit world due to the unborn child inside her. The unborn and children younger than the age of seven are called mizuko, and are considered to belong to the spirit world. For more on this see Yoshiharu Iijima's article, "Folk culture and the liminality of children." Contact me for more info about how to get the full text.
Although anyone in a weakened physical state can be afflicted by mononoke, in the Heian world view women in general are said to be susceptible to the afflictions of spirit possession. Around this time the strong patriarchal influences of China begin to change Japanese society. Under the Chinese-style Buddhism and Confucionism systems women lose their power and are considered to be inferior to men in both physicality and spirituality. This apparent "weakness of will" is what allows spirit entities to take hold of them, which is why women are often used at this time as passive medium vessels for afflicting spirits during divination and excorcism headed by an esoteric Buddhist preist. It is interesting to note that this is in direct opposition to earlier conditions where women fulfilled both roles in their ability to speak for the Gods as powerful shamans and chieftains. See Carmen Blacker's Catalpa Bow for a fascinating discussion of the female shaman in Japan.
Anyway, back to mononoke. The questions was, "Is San is possessed by the wolf gods?" The easy answer is yes, but that has nothing to do with the fact that she is female. There's much more at play here than the obvious of San's circumstances. Let's look at some clues from the dialogue to help us dig deeper (special thanks to Tom Wilkes for his phenomenal translation work).
Yama-inu ni kokoro o ubawareta awarena musume da.
She's a pitiful girl whose heart was stolen by the mountain dogs.
私を殺そうと狙い続けているEboshi says hat San's heart was stolen by the mountain dogs. In the above translation the word heart is given for kokoro, but it is also often translated as soul. San was once human but she was removed from the human world into the kami world when her parents sacrificed her to Moro in order to escape. Moro didn't eat her but instead kept her and raised her in the forest as her own. Eboshi's statement begs the question of whether or not San's soul was stolen or if she willingly remains. One could argue that is more a case of kamikakushi rather than of spirit possession but that's a discussion for another day.
Watashi o korosou to nerai tsuzukete iru
She continues to watch, trying to kill me.
The mononoke are actually all of the kami in the movie, not just the wolves. The mononke in the story don't just possess San they possess all of the forest and the animals within giving them God-like power. They are the collective primordial soul of the forest. Lady Eboshi intends to "tame" the kami by killing the forest God. By removing this spirit all the animals and trees will revert to their mundane states.
Furui kami ga inakunareba, mononoke-tachi mo tada no kemono ni narou.
If the old gods are no more, then even the Mononoke will become mere beasts.
Mori ni hikari ga hairi, yama-inu-domo ga shizumareba, koko wa yutakana kuni ni naru.
If light enters the forest and the mountain dogs are calmed, this will become a bountiful land.
【さすれば】もののけ姫も人間に戻ろうThe statement of light entering the forest is key. It is both figurative and literal. Kami are said to dwell in dark places. By cutting down trees so that light can fall on the ground the land can be cultivated for agrarian purposes. Rice can be grown and the land will become bountiful. No more will the village be dependent on outside sources to feed itself.
[Sasureba] Mononoke-hime mo ningen ni modorou
[If we do so,] the Mononoke-hime may return to being human as well.
Additionally, calmed could be traded again for tamed. Once the animals are no longer Gods they can be hunted and killed by humans. The last part about San returning to human form as well may have been Eboshi trying to subtly manipulate Ashitaka into cooperation. Eboshi may be shrewd enough to see that Ashitaka is taken with San. On the other hand it may be that she looking forward to subduing San and removing another spirit that stands in the way of developing the forest. Or even still, the statement could be subconscious conflicting motivation for Ashitaka to help the humans in order to bring San back into his world. Regardless ambiguity abounds, especially surrounding the use of the work mononoke.
Tanaka's article briefly discusses Miyazaki's use of the term. Little has been said on Miyazaki's part on why he chose mononoke to describe the Gods. Perhaps the fact that he is unwilling to pin that down speaks for itself. Kami are changeable and often ambivalent beings. One can never know them or their will. Even though they are given concrete form by Miyazaki for the sake of the film in the minds of humans they are terrifyingly indistinct and ominous. Perhaps that is why Miyazaki chose to call them mononoke, as it gets back to the earliest meaning of the term outside its application to possessing spirits in Monogatari Genji. As outlined by Tanaka, "Something highly elusive, intangible, and unfathomable."