A Cereberus aka Zombie Dog resembles a Dobermann once mutated from an original dog, it is now a bloodthirsty hell-hound out for anything living being alive or dead or even undead.
Although Cereberus and Zombie Dog are actually two different types of infected dog, the Cereberus was found up in the Arklay forest areas and ran amuck in Resident Evil 0 and Resident Evil 1 while the Zombie Dog is a normal dog infected with the T-virus during the viral outbreak that was in Resident Evil 2.
Anyway you want to call this monster, you can recognize it by the dog like features and the fact that some of its intestines and parts of the ribcage is exposed to the outside. Sometimes the dog's fur would be white and stretched but most of the time it would be red/black ,and sticky to the touch.
They are easily taken down by a gun, stunning them growling before launching another attack aiming for the chest/head area. When fighting a group of them a shotgun is adviced to take them down to avoid their attacks.