How long do y'all think it took for people to forget mammoths? One generation, two, three? They got rarer and rarer, until the clan felled the last one that they would ever kill, and the hunters who were there would, for the rest of their lives, keep telling the story of how they once slayed the most elusive grand beast, that was only seen once a generation. And the youths would listen their descriptions of them, and though the description didn't make much sense - there was nothing else quite alike a mammoth that it could be compared to - they listened and thought that one day, they would encounter a mammoth, too.
They might tell their children and grandchildren of this, how the old hunters would tell them of a spectacular beast that one might see only three times in a lifetime, and perhaps kill just once. It must be true, since the clan still has the tusk of one, but no-one alive has seen one.
Their children and grandchildren would tell their own children only vague tales they used to hear the old folk tell, of grand beasts bigger than horses and bovines, the grandest game of them all, but no-one alive has met someone who has seen one.