Miracle is a Bearded Collie dog that Dean Winchester had adopted.
Background
Miracle was found in Inherit the Earth. He was named by Dean Winchester. Miracle was snapped by Chuck because he wants the Winchesters to be all alone on Earth. Miracle comes back on Carry On where he's shown to be living with the brothers. After Dean's death Sam takes over the ownership of Miracle and takes him to Austin for a hunt. It's presumed that Miracle lived with Sam until his death offscreen during the montage of him finally settling down into an normal life.