Macinwall started in the Brave fandom as a peaceful resolution between the Meriguffin, Meritosh, and Meriwall ships. Originally, fanon depicted Macintosh and Dingwall as good friends who thought Merida and MacGuffin were cute together and tried to ease them into courtship. From that fanon, the idea that Macintosh and Dingwall were both gay and interested in each other naturally progressed.
Eventually, the pairing became fairly popular in its own right, rather than as a hanger-on of Meriguffin. Dingwall is more commonly paired with Macintosh than with anyone else, even in fics where Merida remains single. This allows two rival ships to be easily taken care of, allowing Merida to pursue MacGuffin, remain single, or get together with the crossover ship of the author's choice, which is why it is more popular in the RotBTD fandom than most other Brave ships.
Occasionally, there will be a ship depicting a love triangle between the three suitors. In this case, Macintosh will typically be the aggressive, "bad" boyfriend for Dingwall, while MacGuffin will be the "good" boyfriend. This dynamic is well illustrated in the lone Frozen AU of the Brave fandom, wherein Merida is Elsa and the three suitors are Anna, Hans, and Kristoff>
Note: only post examples from within the RotBTD fandom.