I believe that the first Nika is a person who ate the Gomu Gomu fruit and turned it into a mythical Zoan. That was the ancestor of the Monkey Family with no D in the name
Then comes Joy Boy and eats his ancestor’s fruit and adds a D to his name making him Monkey D
And now Luffy, the great descendant eats the fruit of his 2 great ancestors to fulfill what they couldn’t
Then comes Joy Boy and eats his ancestor’s fruit and adds a D to his name making him Monkey D
And now Luffy, the great descendant eats the fruit of his 2 great ancestors to fulfill what they couldn’t