Well, I have no idea if it is a magical creature, and I doubt it is someone from greek mythology in there.
But the reason I think dates back to when the school was still new:
Hogwarts was built over 1000 years ago by by the four greatest witches and wizards of the age. The four school houses (dorms) are named after them: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin. They build the Hogwarts castle together, far from prying Muggle eyes, since it was an age when magic was feared by common people.
For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprang up between them. A rift began to grow between Slytherin and the others. Slytherin wanted to be more selective about the students being submitted to Hogwarts. He believed magical learning should be kept within all-magic families. He disliked taking students from muggle parentage, believing them to be untrustworthy. After a while there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school.
There is a legend that Slytherin had built a hidden chamber in the castle, of which the other founders knew nothing. Slytherin, according to legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy to study magic.
no subject
But the reason I think dates back to when the school was still new:
Hogwarts was built over 1000 years ago by by the four greatest witches and wizards of the age. The four school houses (dorms) are named after them: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin. They build the Hogwarts castle together, far from prying Muggle eyes, since it was an age when magic was feared by common people.
For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprang up between them. A rift began to grow between Slytherin and the others. Slytherin wanted to be more selective about the students being submitted to Hogwarts. He believed magical learning should be kept within all-magic families. He disliked taking students from muggle parentage, believing them to be untrustworthy. After a while there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school.
There is a legend that Slytherin had built a hidden chamber in the castle, of which the other founders knew nothing. Slytherin, according to legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy to study magic.
It's a pretty grim story to be honest.