https://www.fimfiction.net/group/213500/hellboy

During World War II, a young demon boy was summoned to Earth to upset the balance of the war by German soldiers. But instead, he was rescued and raised by good men of the Allies. They raised him to be a good person and fight for Humanity. It was that very day that the Bureau of Paranormal Research and Defence began. They named this demon child...Hellboy!

But, in an alternate timeline, in the world of Equestria, the evil King Sombra summoned an alternate version of this child for himself. By the time the demon child was brought into the pony world, Princesses Celestia and Luna had arrived with their small army to defeat him. The two rulers had banished Sombra and all was well.

They found the young child, who was transformed into a small red colt with two horns and a giant stone hoof. One of the soldiers suggested that they raise him, teach him all about right and wrong. The princesses agreed.

Since then, the rulers of Equestria had created the heroic organization known as the Supernatural Police Department(S.P.D.) Raised by former Royal Guard, Bold Helm, the young colt grew up to be Equestria's greatest hero. They named him....Hellcolt!

Many years later, Hellcolt teams up with the Mane 6, the Wonderbolts and one of Luna's Night Guard to fight a powerful evil that has come to conquer Equestria. It's time for our favourite demon hero to give the bad guys a little bit of hell!

Cover image is owned by Creepy99 on Deviantart; https://anime-94.deviantart.com/art/Ponified-Hellboy-311802921