In the 2014 rendition of Godzilla, the new enemy is the MUTOs, which are found to have consumed the radioactive energy from a long dead monster of apparent unknown origin.
My initial assumption was that it might have been King Ghidorah, since the skeleton resembled the long neck, but the length threw me off – resembling something more snakelike (which has since been confirmed by the release of the "King of Monsters" trailer).
The original tease for the movie showed a completely separate monster that was intended as the initial opponent for Godzilla – resembling a tardigrade – but was scrapped, and left unlabelled. Again, I thought perhaps, since this monster was scrapped, the skeleton might be of this monster instead, but again, the skeleton doesn't match up (even if this monster could have a skeleton, since it resembles the tardigrade).
So, is there any information about what this skeleton was, before it was killed by the MUTO parasites?