Despite what the reference says, mobs don't appear to have an actual parent_type.
Code Snippet (if applicable) to Reproduce Problem:
mob/verb/Test()
var obj/o = new
world << src.parent_type
world << isnull(src.parent_type)
world << o.parent_type
world << isnull(o.parent_type)
Expected Results:
/mob has a parent_type is /atom/movable.
Actual Results:
/mob has a parent_type of null.