What does Burt Hammersmith compare John Coffey to?

Burt compares Coffey to a dog he used to have. The dog was there when his twins were brought home and played with them and never once snapped. Then one day, the dog turned on his son and tore his face apart. Burt believes that Coffey is like that dog. He seems meek and mild, but can turn on someone like Burt believes he did to the little girls.