User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-5716529-20140920154544/@comment-184612-20140921050749

As for Rusted Kushala in MH4, we found the icon in MH4's data a long time ago, which is why it's listed. We figured it could've been a possible DLC quest like Crimson Fatalis was. We're waiting for confirmation that MH4 is done with DLC before doing anything with it.