Bowser is known as the King of the Koopas, koopas being the turtle-like species of the Mushroom kingdom. However, it is clear that despite having a shell that Bowser can pull his head and arms into, he is not that of a turtle. He has hair, a muzzle instead of a beak, and can breathe fire. On top of this, he is also a lot bigger than any other creature in the Mushroom kingdom which suggests that he is leaning towards that of a Dragon species. His title "King of the Koopas" can simply be that of a title and not a moniker towards his race. He is the ruler, the usurper, the alpha male of the Koopa species thus the title was given onto him
However, as mentioned, he does have a lot of turtle characteristics. He's a reptile, has a shelled body, and can retract himself into his shell whenever he so desires. Returning to the previous argument of being the "King of Koopas"; Bowser could, in fact, be an Alpha Male Koopa, a species of Koopa that has evolved past the usual turtle look. Being the fact that Bowser prefers living in a hot environment over a water environment, it only makes sense that he is a different breed of Koopa. Like you have Sea Turtles and you have Box Turtles. They are both turtles and yet have very different characteristics that set them apart from one another.
We know other Koopas exist. The Fire Brothers and Boomerang Brothers seem to be Koopas that have evolved to stand upright and wear armor. This means that a "Dragon Koopa" like Bowser could possibly exist in that hellish World 8. Thus the King of Koopas can actually be geared towards Bowser's actual species. The Alpha Koopas which involve him, the Koopalings, and Bowser Jr, are a rarer but more powerful and a lot more intelligent breed which allows them to easily rise to power and control all the others.
So let me ask everyone here; what do you think makes more sense? Bowser being descended of Dragons, or Bowser being an Evolved Turtle-Koopa?