Who created the pensieve?

Allegedly it was found on the site of the school's construction.

Traditionally, a witch or wizard’s Pensieve, like their wand, is buried with them, as it is considered an intensely personal artefact; any thoughts or memories left inside the Pensieve are likewise interred with their owner, unless he or she has requested otherwise. The Hogwarts Pensieve, however, belongs not to any individual but to the school. It has been used by a long line of headmasters and headmistresses, who have also left behind their life experience in the form of memories. This forms an invaluable library of reference for the headmaster or headmistress of the day.

The Hogwarts Pensieve is made of ornately carved stone and is engraved with modified Saxon runes, which mark it as an artefact of immense antiquity that pre-dates the creation of the school. One (unsubstantiated) legend says that the founders discovered the Pensieve half-buried in the ground on the very spot where they decided to erect their school.

_{^{Pottermore - Pensieve by J.K. Rowling}}

In reality it almost certainly just carved by a(n unknown) pensive-maker or stonemason and either purchased from school funds when kitting out the headmaster's study or bequeathed by one of its occupants at some point after the school was founded.

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As to its uniqueness, they're apparently rare due to their perceived hazard and the requirement that you be a powerful witch or wizard to use them effectively.

Pensieves are rare, because only the most advanced wizards ever use them, and because the majority of wizardkind is afraid of doing so.

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Inevitably, those with things to hide, those ashamed of their pasts, those eager to keep hold of their secrets, or protective of their privacy, will be wary of an object like the Pensieve.

_{^{Pottermore - Pensieve by J.K. Rowling}}