"and sure enough it was Professor McGonagall who accompanied him"
Witch's Cat
Creative Commons AttributionWitch's Cat - Credit: garlandcannon, Flickr

Professor Minerva McGonagall is an Animagus, able to tranform into a silver tabby cat.

Rowling says this transformation reflects her own personality, although it was considered quite normal for witches to turn into cats.  Cats have long been domesticated, yet remain strong-willed and independent, and they were often associated with superstition, fear, spirits and the moon.

Minerva was the Roman Goddess of Wisdom and Magic, known in Greek mythology as Hera.