I love the idea of Symbiotic Pokemon partners, two different pokemon species that have found a way to work in tandem with one another like Slowpoke and Shelder or my own Sunny the Charmander and Noctis the Gastly. I wanted to come up with more possible symbiotic pairs as well!

I noticed something interesting about Torkoal and Slugma. Some of their pokedex entries mention how reliant they are on heat to stay active. Torkoal constantly consumes Coal to keep it's internal body hot otherwise it becomes slow and sluggish until it's completely immobile, and exposed to the elements Slugma must do all it can to keep from cooling down and becoming hard.
With Slugma housed inside Torkoals shell I imagine the heat of Lava is more than enough to keep it hot, and the shell provides Slugma protection from the elements around it that will help keep it fluid like living in a big hot kiln. I imagine this would make Torkoal faster than it's usual base speed and make both of their fire attacks stronger!