There's no better way to compliment your Harry Potter costume than to dress your dog up as Fluffy. Don't let the name fool you, Fluffy is a ferocious three-headed dog that guards the Sorcerer's Stone... leave it to Hagrid to give such a ferocious beast such a cuddly name. While this would make a cool Cerberus costume on its own it's bound to get more attention as part of a Harry Potter group costume. Source