Bianca Only
If indeed some kind of magic was the cause of this skeleton attacking the horses and ponies, it would undoubtedly be some form of necromancy.
That said, it would be unreasonable to assume that this has anything to do with the Pyre, as this is a ritual site for Varisians in the local area, and was used extensively prior to the founding of Sandpoint.
It is also the burial site of Alika Foxglove, a Knight of Aroden and saviour of the newfound city of Magnimar almost a century ago.
Looking back at the stone archway, you note that one of the doors at the entrance is ajar. Could the work party who had been here to do some cleanup and restoration still be found inside?