They are the Ninja Rats from the anime "Demon Slayer" aka "Kimetsu no yaiba". The mice are the helpers of the sound hashira,Uzui Tengen. They are strong enough to carry one sword per mice. They like to flex their muscles ALOT. They are useful for Tengen since they are able to navigate in small spaces. 
The mice have pointed snouts and large rounded pink ears. Their fur is mostly grey, but the parts covering their legs and hands are black. 
They have a lot of drip on them,just like Tengen.