Chanegling not being fetched by tiamat

changeling is registered as all creature subtypes, yet when Tiamat is cast, it will not fetch any changeling cards even tho they should be registered as dragon subtype. i was also curious is changeling a on the battlefields effect so when its cast it then registers as all creature subtypes, or is it meant to register as all creature subtypes in library and hand, if so then cards like Tiamat that summon X type of creature or creatures that gives mana to X creature in hand should also effect changelings?