Initially thought they were JoJo's Silver Bullet and the IRL NAR racehorse Silver Bullet, but after checking the Skeb commission and quickly glancing through the commissioner's novel it seems like it's just an OC with the same name.

Right is a younger version of the character while racing in Japan, while left is an older version who races overseas.

NNescio said:

Initially thought they were JoJo's Silver Bullet and the IRL NAR racehorse Silver Bullet, but after checking the Skeb commission and quickly glancing through the commissioner's novel it seems like it's just an OC with the same name.

Right is a younger version of the character while racing in Japan, while left is an older version who races overseas.

I tried to read that fanfic on Hameln before, but the number of episodes that thing has gave me archive panic (Warning: TV Tropes link)