Apophis

Apophis is a male serpent.

Origins
Apophis's origins are as yet shrouded in mystery. A unique large black snake with oversized fangs, Apophis was branded by Tom Riddle with the Serpent's Mark and bewitched so that his life would start to wither away if he was too far away from Tom Riddle's Diary for too long a time. He was then gifted to Lucius Malfoy, to whom the Serpent's Mark was keyed, so that he or his son could summon Apophis as an "attack dog" from anywhere using the Snake-Summoning Spell. For years afterwards, Apophis (as he had been dubbed by Malfoy) was kept (barely fed) in the Malfoy's basement, not too far away from the hideout of the Diary.

1993-1994
In September of 1993, Draco Malfoy tricked Hermione Granger into following him to a hidden room under the pretense that he needed her Parselmouth abilities to know what was ailing Apophis (whose health had indeed been failing over the past year, as Lucius Malfoy had lost the Diary to Hermione). Once he'd summoned Apophis and she had begun asking the snake what was wrong with him, Draco ordered him to kill Hermione. Apophis, however, refused to attack his would-be-savior and instead threatened Draco to get him to release Hermione (who he had frozen with the Full Body-Bind Curse).

Hermione then took Apophis under her wing, enlisting Remus Lupin's help to research how to undo, or at least hijack, the Serpent's Mark. She found a home and occupation for Apophis as the new Guardian of the Third Floor Corridor's Fourth Chamber, replacing the Boggart.

In early November, Draco Malfoy and the entire Doom Legion of the Green Peacock (save for Vincent Crabbe) attempted to invade the Corridor. Apophis failed to be of much use as a guardian, as Draco knew a Dark Curse that could be used on Apophis. The serpent nearly bled to death until he was found by Hermione Granger, who fetched Rubeus Hagrid and had him heal Apophis.