Yes, I know this is an old picture, it’s an “Internet Classic”, but still, I love it. Look at the shadow, it looks like Mount Fuji is using an umbrella.
Found this picture at Onigiri Tumblr
This is a lenticular cloud, it’s a type of cloud that is formed only under really rare conditions. It seems that they can form on top of very tall mountains where the difference of temperatures between the top and the base is very high. The temperature difference creates a continuous and symmetrical (in all directions) air flow that facilitates the formation of the cloud with an elliptical form. This type of clouds are usually used for OVNI pictures.
More lenticular picturesin this web.