User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-24897922-20140503192720/@comment-4212769-20140504132939

What I meant is, that MH3U is the expansion of MH3 and has all the monsters of MH3 but MHP3rd has monsters itself that were included in MH3U because they are third gen monsters. Also monsters were excluded in MHP3rd because of the lack of underwater combat and got reintroduced in MH3U. So they had to make another setting for MHP3rd.

About Amatsumagatsuchi: It's only relevant to Yukumo village to fight it because the villagers are affected by its power. So only a Yukumo hunter would be first choice and have the chance to do such a quest. Think of it like that.

About Tigrex and Tigrex Subspecies: They already planned the great show of the Tigrex in MH4 and couldn't afford people getting sick of it so they plugged them out of MH3U.

About the monsters of MHPrd that return and Misty Peaks: They just expanded MH3U with these features too.

MH4U will have again all the monsters that MH4 had as it is an expansion.