The Founding Wizards

"Hogwarts was founded over a thousand years ago - the precise date is uncertain - by the four greatest wizards of the age. The four school houses are named after them: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin.
They built this castle together, far from prying Muggle eyes, for it was an age when magic was feared by common people and witches and wizards suffered much persecution. For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them up to the castle to be educated."
-Professor Binns
Chapter 9, page 150
Harry Potter and the Chamber of Secrets

Hogwarts Crest (left)
representing all four houses.