Answer: Compact Discs

Monster Rancher was a 1997 release for the Sony Playstation that focused on the game play mechanic of raising, fighting, and breeding monsters. The in-game monster generation algorithm used a pseudo-random number generator. The designers at Tecmo realized that because the number generator that seeded all the in game monsters wasn’t truly random, it would eventually begin to repeat stats and the whole allure of the game, breeding and creating unique monsters, would fall apart.

In order to compensate for the lack of true randomness, the game designers came up with a rather clever work around. Players could insert compact discs into their Playstation unit and Monster Rancher would read the data encoded on the disc and use that data as a “random” seed for a new monster configuration. While that alone was cool enough, and led to a large base of fans testing CDs and swapping tips with each other, the design team went one step further.

In addition to the ability to use any CD, the team specified certain CDs as “pandora” discs wherein the disc didn’t provide a random monster but a unique monster specific to that album. For example, if you used the Monster album by R.E.M., you received a dragon named Apocolis. Pop in Bringing Down the House by the Wallflowers, and you got a giant flowering plant monster. Some of the “pandora” discs were particularly clever, like the Destiny album by Gloria Estefan; the monster that spawned was wearing the same dress she was wearing on the cover of the album.

Later releases in the Monster Rancher series kept the CD mechanic and added in support for DVDs. In Monster Rancher 4, for instance, you could put in the Harry Potter DVD to create a unique owl monster.