WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 11, "Tonight," now streaming on Funimation, Hulu, and Crunchyroll.
Things are always intense on Demon Slayer: Kimetsu no Yaiba, but in Season 2, Episode 11, they got very... weird. After Zenitsu Agatsuma goes missing, which causes Tengen Uzui to call off the mission, Inosuke Hashibira decides he's had enough of these undercover games. Launching himself head-first through the ceiling to access the attic, because how else would he do it, Inosuke says a strange few words: "Hey Mice! My Swords!" Cue two overly buff, upright-walking mice who are, indeed, bringing Inosuke his swords.
The aptly named Muscular Mice are exactly what they sound like -- very well-built rodents. Animal assistants aren't uncommon in Demon Slayer, as most Corps members are accompanied by a Kasugai Crow that delivers orders from Ubuyashiki. However, Inosuke tried to eat the crow assigned to him, so he clearly doesn’t work well with animals. Yet, perhaps unsurprisingly, he is totally enamored with the Muscular Mice and wants his own miniature minions to boss around.
The Muscular Mice are servants of Tengen Uzui, and it appears he has temporarily gifted a pair of them to Inosuke for their mission in the Entertainment District. The Mice clearly belong to Tengen -- rather than the Demon Slayer Corps -- given that they are each wearing gemstone-embedded headbands identical to the Sound Hashira's. Although he dubs the Muscular Mice "The Shadow Demon Slayer Corps," it feels highly unlikely that anyone else considers them actual Corps members.
It's implied that the Muscular Mice are no more than average field mice that Tengen has trained to act as his "ninju." The flashy Hashira demands equal theatrics from them, and only accepts mice who will be narcissistically proud of their outrageous musculature. In point of fact, "muscle" appears to be the only word the mice actually know how to say. How Tengen gets these mice so buff, walking on two legs, understanding human language and even talking is a mystery on par with Muzan Kibutsuji's origins, but is probably less likely to ever be explained.
Pumped-up mice who work behind the scenes to help Demon Slayers slay demons are far too bizarre to be introduced for such an insignificant task as delivering swords. Tengen mentions during the Taisho-Era Secret segment that he brings the mice to fit into small places where he can’t go. Daki, Upper-Rank Six -- whom Tengen has been searching for -- has already shown she can escape through the walls and ceilings inside the Entertainment District houses. If she were to attempt to flee like that again, it may be none other than the Muscular Mice who can track her down.
The Muscular Mice have tremendous strength beyond what even their cartoony muscles should be capable of, and while they probably still can't fight a demon, they might just find a way to contribute regardless. As Tengen Uzui's flashy proteges, these mice will probably be around for the majority of the "Entertainment District" arc, as they even briefly appear in the opening credits. The Muscular Mice may not defeat Daki themselves, but it's amusing nonetheless to think these weirdo rodents could have a major part to play as Demon Slayer continues.