Yes, the Runespoor Patronus is considered extremely rare because the Runespoor itself was a very rare magical beast found only in the forests of Burkina Faso in Africa. Furthermore, Runespoors were associated with Dark Wizards and Dark Magic as they were serpents, and only Parselmouths could communicate with them. Therefore, the chances of there being a Runespoor Patronus would be very rare as a Patronus heavily relies on not being Dark, as the very being it has to fight is a Dementor, which is a Dark being.