User blog comment:Kogath/Monster Hunter X Discussion/@comment-6050823-20160122020751

New Info About Osutogaroa (Info comes from MH Festa 16)
 * The large cave within the Ancient Forest, where the Wyvern Graveyard is, has a large body of water. This large body of water comes from the expansive sea that the cave is connected to.
 * This explains how Osutogaroa got to the Ancient Forest and how it got certain parts from monsters not found in the Ancient Forest, at least thus far.