reminds me of crafting compressed cobblestone in modded minecraft. all this is missing is a rat dimension portal that you make by crafting all of the rats together.
this would make a great mobile game if you expanded this a bit.
maybe add a few other mechanics, like instead of just getting a rat by clicking the button, you get a random assortment of new rats daily, to keep people coming back. And give them not just peasant rats, give them a random assortment from what they've unlocked. And maybe you could leave special food out to attract certain rats.
oh also, i think this would benefit from adding crafting recipes for different rats. like maybe farmer + peaseant = a new type of rat. you could build a deep crafting system if you had some sort of logic behind what gets crafted.
i love the rats, could use some polish though. for example, if i click an area where two rats are standing, both of them get clicked.
I got up to scholar rat before i stopped.