Fallout: Equestria - Burning Earth (written by Endless Madness on FiMFiction) is an ongoing spin-off of Kkat's Fallout: Equestria. As of Chapter 14, there are over 21,000 words.
Burning Earth revolves around three characters: Fizzy Pop, a unicorn from Stable 6 who managed to escape into the wasteland after raiders invaded her home; a stallion named Dirt, who was mugged and seeks to reclaim what was rightfully his; and a Dashite pegasus named Night Shade who is trying to start a new life for herself. The story follows their tales as they eventually meet, and the bonds that they forge over the course of their adventure.
The link to its FiMFiction page can be found here.