Bratworst the Mustard Firing Sausage Dog Sidekick is a character in Moshi Monsters and is the companion of Commander Sassafras. They first appeared in issue 25 of the Moshi Monsters Magazine, as an illustration with the Missing on a Star story. However, they do not appear nor are mentioned in any fiction surrounding "Missing on a Star".
Their first actual appearance is in the Katsuma Unleashed game as the World 4 Boss, where the player battles them before battling Sassafras. They show to have a peculiar way of running, using their hind legs in such a way that their lower body comes first and their upper body leans backwards, joined with one of the classic Hanna-Barbera "Cartoon Running" sound effects. While trying to bodyslam you, Sassafras is in the background cheering Bratworst on, jumping up and down and making faint boxing movements. There is also a boxing bell sound every time either you or Bratworst endures a hit. Interestingly, Bratworst has an evil laughter that sounds very much like Dr. Strangeglove's laughter. The 3DS version of the game shows more clearly that they have a C.L.O.N.C. logo marking on their hip.
In the files of the Katsuma Unleashed game, the files for them are listed as "Bratworst" thus confirming their name. Outside of this and the tweet below, Bratworst has never had a direct reference to their name in any form of media or fiction.