Mine has come up from an idea I had as a 13 year old, way back in the early 2000s.
Originally my main character was just a Sonic OC. Then the concept for the story changed as I aged, and finally in 2009 I had an idea for a Battle Shonen with a dinosaur mc of epic proportions.
Then the story continued to evolve for another 7 years until I finally, finally after many variants and failed starts I buckled down and decided to take the plunge into it. And in January 2017 I finally started and stuck with it and built up a buffer and released it in August 2017.
As for how I came up with it, it's just a mixture of my interest in dinosaurs, action cartoons, and heavy metal music.