Mongoose's Slayer's Guide to the Undead had some interesting material concerning the 'origin' of undead creatures - mostly having to do with Orcus, who 'rewards' evil with the 'gift' of undeath.
http://www.mongoosepublishing.com/detail.php?qsID=48&qsSeries=Slayers%20Guide
Devourer's for instance, can arise spontaneously from the souls of people who 'fed' upon onthers in life - through manipulation, emotional blackmail, profiting from other's suffering etc. Upon death, some such souls manage to 'escape' into the Astral, where they eventually adopt the form of a Devourer and feed on the life-force of others. Others are created by Orcus directly, etc. etc.
Similar origins are postulated for most of the types that later 'Spawn' new creatures. A form of negative energy infection...
Amal