User blog comment:BannedLagiacrus/Discussion of the Week: Extinct and Unknown Status Monsters/@comment-17735225-20140531161226

I believe all of these monsters are scrapped monster ideas from the very early development phrase. Techinically they are 'ancestor' to modern monsters in game because they appear in paper before 'evolving' to become the current roster.

But that doesn't mean that they are entirely scrapped. IIRC wyvern rex was in the concept section of MH1(correct me if I'm wrong) and it finally made it as Tigrex in MH2F. Leviathans made it in MH3 (even though pretty different from what it is).

I believe they are more concept monsters that didn't get their 'lore' explained since they didn't fit at all in the current game universe. One is the gigantic mechanified dragon and another is a giant turtle with a city on its back. I'm pretty sure Banned has their pictures. Mine was lost long time ago.