Appearing of Weird Cornered Cloud in Sky?
The left edge of the cloud is a real edge, irregular and fuzzy as expected. However, the lower sharp edge is just an illusion due to illumination. Just before sunrise or after sunset, the sun can illuminate the upper clouds but not the lower ones. If a cloud happen to be in the limit between illuminated clouds and shaded clouds, it will have its upper side illuminated and the lower one in the shade. That is what you are seeing, the lower edge is not the edge of the cloud, is just the limit of the illuminated part of it.

If you pay attention you can follow the contrast in illumination even outside of the cloud to the left. Where the illuminated air above the line have a glow not visible in the part under the illuminated part.