Cerberus:

Most members of the canine family are such hearty extroverts that they are not favoured for thaumaturgy or the Black Arts, although there have been some notable exceptions. Most famous of underground dogs is Cerberus, the three headed dog who guards the entrance to Hell. He has to be placated with gifts before he admits dead souls to their eternal home. But even Cerberus has a softer side as Orpheus and The Lord of Misrule demonstrated when they charmed their way into the underworld by playing to him on a lyre.

Hint: For one with such a bad reputation, it's rather hard to believe that he has a sweet tooth.