Me & My Husband Frankah Are In Onigashima and we saw this dungon which has an unknown name and were wondering why there is an extra dungon in onigashima?

I think it's under construction for new/extra content (maybe in next major update)
I haven't been in Onigashima in a while but when I last went there and saw it, there's no portal but there was a Japanese Under Construction sign.

Thank You Karen ^^

where ? o.o screenshot pls XD

There was never an actual dungeon there in Onigashima, only a dungeon icon on the map which was removed shortly after.