I do absolutely nothing with Seramas, so I did a quick search of the Serama thread here. Creeper legs come from a gene that causes very short stunted legs when the birds are het for it, and is lethal when the birds are homo for it (so breeding two creeper legged birds would be fatal for the offspring). I am sure you could get further info there. Apparently the legs are so short the wings cannot be held properly for show.
Edited by SpringPeeper - 1/9/16 at 5:47pm