i got curious about this too so i did some research! i think it’s because mouse-sensei is a tesso yōkai (originated 1000 years ago), compared to most animal yōkai (usually comes in packs) tesso is a "singular" yōkai. There is only one tesso, based on the mythology he (tesso) is bound to the spiritual realm by his curse and his desire to destroy the rival enryaku-ji monastery. i’m not sure if i explained this well :,D
ooh that sounds so interesting! I'll try and get readings about this too, thank you!
Just my theory so it's not canon but it's probably related to their "origin". Mouse-sensei born from human grudge like how most yokai said to be fallen human(or god) while Hatanaka and gangs mostly just animal live long enough to become yokai, even from human vs animal we know human has longer lifespan.
I just keep wondering, how is it that animal yokai's have shorter lifespans compared to other average ones, yet mouse-sensei's been around since the early days of school? did i miss something? they make it sound like hatanaka could just drop dead one day because he's an animal yokai, but i remember that it wasn't until kai started research on prolonging life expectancy that they were able to help lengthen them