User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5020152-20121014204849/@comment-4212769-20121019002446

Or the MH4 region is just in between the both other regions and has land bridges to both. That's how some monsters came over. Of course those who fly just fly to there and those you swim have no problem neither getting to there.

With Kut-Ku being a subspecies you're probably right as it has one toe more like Rathalos and Rathian having subspecies in Moga and Minegard but Tigrex could just be always able to do this but doesn't have the opportunities in other regions. Though the area seems quite unlikely for a normal Tigrex.