Here is a video I made using what has emerged over the years regarding the Battersea Flying Pig.
More info from Wikipedia :
The original Pink Floyd pig, a 12-metre (40 ft), helium-filled balloon, was designed by Roger Waters and built in December 1976 by the artist Jeffrey Shaw with help of design team Hipgnosis, in preparation for shooting the cover of the Animals album. Plans were made to fly it over Battersea Power Station for a three-day photo-shoot, with a marksman standing by to shoot the pig down if it broke free. On the first day, poor weather and delays meant the pig was not launched, and the marksman was told he was not needed.
On the second day, 3 December 1976, the marksman was again not present because no one had told him to return. A gust of wind tore the pig loose. It disappeared from sight within five minutes, and was spotted by airline pilots at thirty thousand feet in the air. Flights at Heathrow Airport were cancelled as the huge inflatable pig continued eastwards across flight lanes and out over the English Channel, finally landing that night on a farm in Kent, where it frightened a herd of cows.
The pig was recovered and repaired for the resumption of photography for the album cover, but the sky was cloudless and blue, thus "boring". However, the pictures of the sky from the first day were suitable; the album cover was created using a composite of photos from the first and third days.
The pig that was originally floated above Battersea Power Station was called "Algie".