I managed to take a few screens of it, it's hard to really make anything out, but it does look like a destroyer..
It even has the same type of cracked skin along it's back that the destoryers have with all the fire coming out.
Remember one of the official GW:EN trailers, at the end there was a huge Destroyer in a pit of lava, spewing fire from it's mouth all over the place! It sort of resembles the thing in the lake!