Often dull, often about space, often sexual.
I recall a dream fragment where I was looking at a GIF on Imgur where there were a number of horses in the picture standing over wet, leaf-covered ground. A horse suddenly plunges into the ground. The other horses wandered around cluelessly before some started digging on another spot in the ground and uncovered a pool which the previous horse comes out of, revealing that the horses were standing on an island of dead leaves.