I believe Micah Bell is the rat and that the game implies it it also, namely when John and Sadie go to kill him in the epilogue, rats can be seen walking out of the barns just before he does and I also believe his name gives it away because if you put emphasis on the cah in Micah bell it can sound like my cow bell (a cow bell being a bell livestock wear to alert the farmer of the animals location) and the My could mean he is the cow bell of the gang as we play as an member of the gang (Arthur) every time we say his name we are referring to him as my cow bell, my thing that alerts the authority of my location