Did you see vishnu

maybe 

No that was another animal. They're called shinomura or swarm of death! They were a species of muto and appeared just after Hiroshima. Oh btw, I wouldn't call her Vishnu either, some people don't like it.

Yeah, let's refrain from using "Vishnu" as the  moster's name.

SPOILERS

it makes no sense for that to be shinomura, especially if you read the graphic novel. Shinomura is made of several smaller creatures forming a single organism. Whats in the movie is a large skeleton of a single organism. How could a swarm of several smaller creatures make a huge skeleton, it cant so stop saying that. Also shinomura was evaporated in an atomic blast. The skeleton in cave is something we have not seen  and I would guess we are not likely to see.

I agree with DEADPICKLE. It can't be Shinomura. It has to be something else.

@M_U_T_O - I think you could be right! That is a very interesting theory you have presented to us.