An inflatable pig. I cheated MT - but luv this excerpt from Wikipedia:
The giant, helium-filled pig seen on the cover was actually flown over Battersea Power Station for the photo shoot. On the first day of shooting, a marksman was on hand in case the pig broke free. On the second day of shooting however, photography commenced before the marksman had arrived. Ironically, a gust of wind broke the pig free of its moorings. Because there was no-one to shoot the pig down, it sailed away into the morning sky, with the photo crew watching in horror. A passenger plane reported the pig, causing all the flights at London Heathrow Airport to be delayed. Legend has it that the pilot was breathalysed upon landing. A police helicopter was sent up to track the pig, but was forced to return after following the pig to an altitude of 5,000 feet. A warning was sent out to pilots that a giant, flying pink pig was loose in the area. The CAA lost radar contact on the pig near Chatham in Kent, at a height of 18,000 feet and flying east towards Germany. It finally landed in a farmer's field (without any damage), where the farmer was angry because it scared the sheep. The resulting pictures were not deemed suitable, and the final image is a composite of the power station picture from day one and the pig from day two. While Roger Waters had never intended for a composite picture to be used, it was the cloud coverage over Battersea Power Station that ensured a superimposition would be necessary.
Similar inflatable pigs have since featured in Pink Floyd concerts.
