Why was Amaterasu so angered by the flayed horse that Susanoo threw at her?

Question

One rather memorable event in the Shinto creation narrative is that Amaterasu retreated into her cave (the Ama-no-Iwato) after Susanoo did a bunch of awful things to her. One of the things Susanoo did was throw a flayed horse at her.

Now, I can see why anybody would be unhappy about having a flayed horse thrown at them, but I can't help but wonder if there's some subtext here that I'm missing. Why specifically a flayed horse? For example, was it the case that the horse was an animal sacred to Amaterasu? Or, contrariwise, was the horse perhaps seen as an "unclean" animal, whereby having one thrown at you was some sort of grievous insult?

Answer 1

It wasn't the flayed horse that Amaterasu-ōmikami was offended by, per se.

According to the kojiki (古事記), what happened was that Susano'o flayed a beautiful spotted horse and threw it into a room where Amaterasu's weavers were making clothes. In the ensuing shock and chaos, one of the weaving girls was impaled on a shuttle (through her private parts...) and killed.

The version in Nihon Shoki (日本書紀) is similar, but elaborates that the unfortunate weaver was waka hirume no mikoto (稚日女尊), Amaterasu's own sister. Understandably, Amaterasu was less than amused. The Shoki further explains that, in her grief and anger:

【日本書紀　卷第一　第七段】 故天照大神謂素戔嗚尊曰　汝猶有黑心　不欲與汝相見　乃入于天石窟

(Nihon Shoki volume 1 section 7) Therefore, Amaterasu-omikami said to Susano'o; "You have a black heart; I do not wish to see you." And so she entered Ama-no-Iwato.

So it seems that Amaterasu was angered by the consequences of her brother's actions, rather than the horse itself.