To write an engaging MLP and Red Dead Redemption fanfic, first, blend the unique elements of both worlds. For example, you could have the ponies in the Wild West setting of Red Dead Redemption. Consider the personalities of the MLP characters and how they would react in the tough and lawless environment. Also, focus on creating a good plot. Maybe a group of ponies are on a quest to save their homeland but have to deal with outlaws and the harsh conditions of the Wild West.
Start by researching both MLP and Red Dead Redemption. Know the lore, the characters, and the settings well. In your fanfic, you can introduce MLP magic into the Red Dead world in an interesting way. For instance, a unicorn pony could use its magic to solve some of the problems that the characters in Red Dead usually face, like finding hidden treasures. Make sure to give each character a distinct role and development arc in the story.