Rolling Stone has an in-depth article explaining why Pink Floyd adopted a flying pig as a symbol, and how it became the most famous rock and roll prop ever. The pig made its first appearance on the cover of the 1977 album Animals. Roger Waters tells the story of the photo shoot:
The pig has been escaping ever since. The latest incident was at the 2008 Coachella festival. Link -via Metafilter
The first day was that beautiful sky, but the pig escaped. The rope broke and it drifted off, up into the flight path at Heathrow. Then the next day, we flew another pig, and it was a bright blue sky, and so the photographs weren't nearly as interesting as they had been from the day before. So in fact we stripped the pig from the second day into the photograph from the first day that didn't have a pig in it because it had already escaped. And that is what appeared on the album cover.
The pig has been escaping ever since. The latest incident was at the 2008 Coachella festival. Link -via Metafilter