Lucky

He is a skinwalker posing as a family dog called, "Lucky". His real name is unknown.

Background
He is part of a sleeper cell ready to turn his family on the signal. Before he was a skinwalker, he used to be a homeless person. He later entered the home of Amanda Duren and her son. He eventually began to start caring for the family he was meant to turn. So he began killing people who threatend the family.

Sam and Dean investigate the murders and find out its Lucky. When Lucky changes back into his human form. Sam follows Lucky, and Lucky is hit by a car and sent to the pound. There Sam and Dean corner him and take him back to their room. Back in the motel room (tied up with silver rope), they question him and Dean figures out that Lucky care for the family. So they get him to call the Alpha for them.

So Lucky acts as bait, but their plan is foiled when they bring Lucky's family with them. When the Winchesters begins their assault, Lucky helps them and locks them into a room for their own protection. When one of the skinwalkers try to hurt his family he attacks, and is injured. After he revealed himself to Amanda, she doesn't want anything to do with him. So Lucky heads off on his own.