"sth" means some thing! you didn't know?
it is just a short of that like "sb" which means some body!
are they really English or we made them?
your right the harry potter and narnia are completely different in their story but the creatures are mostly save like cantor(I don't know is it right or not because I have never seen their names in English)!! you mention that the harry potter series show you don't need god but I don't think so , of course there is no mention to God but I think the writer didn't want her story be religious! because she is christian and believe in God! and another reason I think it is not is that she mentioned to love a lot and the power of that and I don't think love is against God!
but maybe you are right I I am making mistake
by the way thanks for your helping <3