User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-74.193.211.56-20121106231625/@comment-4212769-20121108192253

In the Old Jungle Plesioth is in a river. Rivers always go forth to the ocean. With his lungs it can survive in salty waters. And from there it can get to the Deserted Island and the Flooded Forest.

So the same is possible for Lagiacrus in the Unite areas.