User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-4694695-20130828165613/@comment-16545476-20130828180657

Technically speaking, every new game in this series only adds in about a dozen or so new monsters each time. So it's reasonable for Capcom to reuse old monsters in newer generations if they want the playtime to last longer.

They did start to make an effort to link every generation into the same universe, so players like us would feel quite odd if some older species of monsters just went "poof and extinct" in newer generations. You have to give them credit on that.