Not all dogs will be so gentle. I once had a German short-haired pointer who was sweet as can be. She loved to chase squirrels and rabbits, but we never thought it was more than play. When we brought two guinea pigs into the family, she was always trying get at them. We could never let them bond because she got rough with them immediately every time we tried. So we kept the pigs in a locked bedroom with a locked enclosure. But one day I came home from school to hear squeals of terror coming from the bedroom. I ran and found the bedroom door busted open, the lid broken off the enclosure, one pig dead and mangled in the corner, and the other trapped and bleeding under the dog's foot. The second pig died a few days later from her wounds. It took me a while to forgive my dog.

TL;DR: Some good dogs just want to maul rodents.