Founder : Salazar Slytherin
Professor Salazar Slytherin was a pure-blood wizard of the medieval times. He was a Parselmouth (a wizard with the rare ability to speak to snakes) and skilled at Legilimency. He was one of the four founders of Hogwarts School of Witchcraft and Wizardry and the former best friend of Godric Gryffindor, a fellow founder. Slytherin didn't trust Muggle-born students, and he expressed his opinion by proposing that they should not be accepted at Hogwarts. However, when the other Hogwarts founders didn't agree, Slytherin created the Chamber of Secrets, concealing a basilisk inside. He left the school soon afterwards, never to return and died at some point in the Middle Ages. Like Rowena Ravenclaw and Godric Gryffindor, Salazar Slytherin appears to have carefully selected members of his own House. According to Albus Dumbledore, the qualities which Slytherin prized in his "handpicked students" included resourcefulness, cleverness, determination, and a certain disregard for the rules, along with the ability to speak Parseltongue. He also selected his students according to cunning, ambition, and blood purity. His House within Hogwarts is symbolised by a serpent, with House colours of green and silver.
Animal : Snake (Serpent)
A snake is a long, thin, legless reptile. The symbol of Slytherin house is the serpent, and the dark wizard Lord Voldemort had a pet snake named Nagini. There are many types of snakes in the magical world, but the biggest and most dangerous of them all is the Basilisk also known as the King of Serpents. Voldemort's mark, the Dark Mark, is a skull with a snake coming out of its mouth. Snakes speak a language called Parseltongue. Only a select few wizards can speak this language, most of whom were Dark wizards. They are called Parselmouths. Some known Parselmouths are Salazar Slytherin, Marvolo Gaunt, Morfin Gaunt, Merope Gaunt, Lord Voldemort, Ginny Weasley (while under the influence of Tom Riddle's Diary), and Harry Potter (although he lost the ability later).
Colors, Traits and values
Colors : Green and Silver
Slytherins tend to be ambitious, shrewd, cunning, strong leaders, and achievement-oriented. They also have highly developed senses of self-preservation. This means that Slytherins tend to hesitate before acting, so as to weigh all possible outcomes before deciding exactly what should be done. According to Albus Dumbledore, the qualities which Salazar valued in the students he chose included cleverness, resourcefulness, determination, and "a certain disregard for the rules." Dumbledore noted that all of these were qualities possessed by Harry Potter, who was in Gryffindor. Contrary to the wishes of the other founders of Hogwarts School of Witchcraft and Wizardry, Salazar Slytherin wanted a stricter policy of admission to the school —one limited strictly to pure-blood students. The controversy this created apparently led to him leaving the school and the Chamber of Secrets behind. A millennium later, the house's students are still considered affiliated with Salazar's reputed views, and open expression of pro-blood purity sentiment does seem to be more common in Slytherin than in other Houses. This disagreement over blood purity, which caused Salazar Slytherin's friendship with Godric Gryffindor to break, is the origin of the feud between Slytherin and Gryffindor, the two most competitive of the houses.
Ghost : Bloody Baron
The Bloody Baron (late 10th century – early 11th century) was a wizard who attended Hogwarts School of Witchcraft and Wizardry in the Founders' time and was Sorted into Slytherin house. After Hogwarts, he fell in love with Helena Ravenclaw, but she did not love him. He was sent by Rowena Ravenclaw to find Helena in Albania, but Helena refused to return with him and so he killed her in a rage. When he realised what he had done, the Baron stabbed himself with the same knife and died. After death, he is one of the ghosts that haunts Hogwarts School of Witchcraft and Wizardry, and the ghost of Slytherin House. Covered in Helena Ravenclaw's blood to this day, he earned himself the moniker of "Bloody Baron". At the age of eleven, the Baron became part of one of the first generations of students at Hogwarts School of Witchcraft and Wizardry in Scotland, which, at that time, was still being run by its original Founders. He was Sorted into Slytherin House and was educated in the magical arts by Salazar Slytherin for the seven years that followed. For unspecified reasons, presumably related to his apparently violent, dangerous nature, the Baron has gained Peeves's respect and is the only one, besides Professor Dumbledore.
Head : Severus Snape
Professor Severus Snape (9 January, 1960 – 2 May, 1998) was a half-blood wizard who was the son of the witch Eileen Snape (née Prince) and Muggle Tobias Snape. During his lifetime, Severus Snape was Potions Master (1981–1996), Defence Against the Dark Arts professor (1996-1997), and Headmaster (1997-1998) of Hogwarts School of Witchcraft and Wizardry (which he attended as a student from 1971 until 1978). He was also member of the Order of the Phoenix and played a very important role in both of the Wizarding Wars against Lord Voldemort. Severus Snape was raised in the Muggle dwelling of Spinner's End, which was in close proximity to the home of the Evans family, though in a poorer area. He met Lily and Petunia Evans when he was nine, and fell deeply in love with Lily, becoming a close friend of hers. In 1971 he began his first term at Hogwarts School of Witchcraft and Wizardry where he was sorted into Slytherin. This put him in the same year as his true love, Lily Evans, but unfortunately for him, in rival houses. Severus was made a member of the Slug Club, presumably because of his brilliance at potion-making, and Horace Slughorn kept a picture of him as a student, clutching his copy of Advanced Potion-Making. Despite this, Horace did not have many hopes for Severus' future, as his photograph was kept behind many others. Shortly before Lily Evans was murdered by Lord Voldemort, Snape changed sides and became a member of the Order of the Phoenix and double agent during the Second Wizarding War. With tremendous difficulty, Snape was able to prevent Lord Voldemort from learning the truth about his loyalties. Despite the opinions of most others including Harry during his early life, Albus Dumbledore trusted Snape for reasons that were kept between them both until their deaths. The relationship between Dumbledore and Snape would be one of an unusually strong loyalty, so much so that Snape agreed to kill Dumbledore upon Dumbledore's own request.
Slytherin Common Room
The Slytherin Dungeon, also known as the Slytherin Common Room, serves as the common room for students in Slytherin house at Hogwarts School of Witchcraft and Wizardry. The common room is a dungeon-like room with greenish lamps and chairs. This dungeon extends partway under the lake, giving the light in the room a green tinge. The common room has lots of low backed black and dark green button-tufted, leather sofas; skulls; and dark wood cupboards. It has quite a grand atmosphere, but also quite a cold one. The password to the common room changes every fortnight, and it is posted on the noticeboard. It is decorated with tapestries featuring the adventures of famous Medieval Slytherins.
Entrance
The entrance to the common room is located behind a stone wall in the dungeons of Hogwarts Castle. A password is required to enter it; whereupon a passage is revealed leading to the common room.
Reputation
It is said that all Dark Wizards were in Slytherin (though this is untrue, as Peter Pettigrew was a Gryffindor and Quirinus Quirrell was a Ravenclaw), though how many were from this house before Tom Marvolo Riddle began recruiting Death Eaters there is unstated. As Slytherin was his own House, it was certainly most easy for him to recruit there. It might also be possible (though unlikely) that the Sorting Hat simply sorts anyone with ill intentions into Slytherin, regardless of whether they possess cunning and ambition. However, it must be noted that Slytherins who oppose use of the Dark Arts and discrimination based on blood purity do exist, such as Horace Slughorn and Andromeda Tonks; the latter was related to many different Dark wizards and witches (most notably her sister Bellatrix Lestrange, her brother-in-law Lucius Malfoy, and her nephew Draco Malfoy), but chose to forsake the loyalty to her family in marrying the Muggle-born Ted Tonks. Also, the allegiance of Severus Snape was found to be ultimately towards Albus Dumbledore, serving as a triple agent for the Order of the Phoenix. Furthermore, there have been Dark wizards from other houses; Peter Pettigrew, who betrayed James and Lily Potter to Voldemort, was a Gryffindor.