Wasn't sure where abouts to work this into the article, but in the Gunslinger Born #5 we're told that Gan originally chose these particular 12 animals as his guardians because they were his "most loyal servants". He put them in place to guard the 12 main doorways, after Maerlyn's mirror had shattered and it's shards had blown through into all other worlds, to prevent such a thing from happening again.