I've sort of covered it before.
I believe the original Nika manifested in a slave of the First World that worked in the mines to feed the Mother Flame, which had such a profound impact on the environment it created the source of Devil Fruits.
The hopes and dreams of slaves yearning to be freed from their chains is what birthed the Devil Fruit, perhaps the first, that manifested Nika.
The battle for the First World basically resulted in the birth of a Second World with many new species birthed from the mutations of the original Mother Flame. The Giants, due to the slow ageing, held on to Nika the longest in tradition that was also passed down to the Buccaneers that descended from them.
I think Nika originated from hopeful stories told by slaves over generations, and eventually manifested when Devil Fruits came into being.
The stories were probably made up for children to give them hope and that's why Nika is a monkey with fire-like hair and rubber body that he uses as a drum to make music and spread joy, laughter and liberation. He's probably the Sun God because the slaves working mines that rarely saw the sun came to think of it as a hopeful symbol.
This manifested in a Devil Fruit, possibly the first, and the consumer of it was a slave who would be known as Nika from the stories.
Luffy losing control due to his injuries and exhaustions is why the DF took over and he is now the Nika from the stories, but mastering the DF will not confer a transformation into the ape like creature, that's why I don't think we'll see Luffy in this form again.
tl;dr the mythological Nika was an ape. The first DF user who was known as Nika as well was just a human, and so was Joyboy (don't let the giant Strawhat fool you), and this ape form only happens when the human of the DF loses control.