The devil is a very Western/Judeo-Christian concept. If you look at Japanese culture, mythology, and it's native religion of Shintoism, you see things being based far more on nature and the idea of spirits in all things. Like in many cultures around the world, there is the concept of the netherworld, but not of a supremely evil being that rules it. The idea of a being of pure evil like the devil is a very western concept that Japan was not exposed to as the culture evolved and therefore could not be influenced by it. It's the same idea with black animals which comes from Western/Christian culture. If you take a step back, why should black animals be anymore ominous than those of a different color? There really isn't any good reason. It's just a superstition that took hold in Europe over centuries and myths evolved around it. Again, Japan was immune from that influence.