Apparently they only have the rights to Godzilla himself. Non of the other monsters. So we won't see Rodan, Mothra or anything else. The other monsters are going to be generic ones that likely pay homage to the other big names.
I've been arguing this for a while. If it's a super hero we have seen in film recently or is fondly remembered audiences then there is little reason to waste time with an origin story. Superman, Batman and Spider-man are all great examples of this and retelling their origin feels more like a drag on the limited screen time then anything remotely interesting.I don't have a problem with it for non-established heroes in film. In fact there it is certainly needed.