For all intents and purposes, Riot is a dog. He barks like a dog. He eats like a dog. He follows Sam Winchester around at all times like a dog.
In reality, Riot's more than Sam and Amelia's dog. He's a skinwalker, cursed with the ability to shift between canine and human form and never truly fitting in as either. When he decided to follow Sam out of the house one night and to an abandoned cabin in the woods, he just wanted to bring his owner home. He had no idea he accidentally stumbled into a hunter's layer.
But, like any good dog, he sticks around, hell bent on keeping his human and his pain-in-the-ass older brother alive, even if it kills him.