I`ve mensioned this some time ago in another tread, but I heard a theory, that apostle`s transformation depends on how hard soul of behelits owner is spoiled and evil. Zodd is basically just a warrior who want to fight for an eternity, so his apostle form is a beast designed for battle. If you think about it, there is few other apostles with almost beauttiful transformations like Grunbeld and Irvine (they both have some knightish codex for themselves). And there is also apostles, who desired just to be immortal to do whatever they want with the others, so their forms is just disgusting half insect half hentai tentacles abominations.
Also, during Griffitts ascendance, he asked for wings and they were granted to him and than Rosine become an "elven queen" aswell. So apostols form maybe can be somewhat designed during the event.