The Dead rat is collectible item in Resident Evil 7: Biohazard. It is a carcass of a rat.
It has no game changing purpose other than being a collectible. To "collect" them the player only needs to interact with it. Successfully finding all rats will reward the player the award "Ratcatcher".
There are three dead rats in "Bedroom". They only appear after Marguerite Baker serves the second dish. Another can be found in the main game, hidden in a bin in the old house.