The woman, from Castleford, was training for a charity hike at the site when she spotted the strange apparition on the evening of March 7 as she passed the front of the castle, near Wakefield.
She took a photo of a large, dog-like creature on her mobile phone and later sent the image to Haunted Yorkshire, a paranormal investigations group.
Local authors Richard Bramall and Joe Collins, who run the forum, believe the spooky hound could be a 'cryptid' - a creature whose existence is unverified. Famous cryptids in folklore include yetis and the Loch Ness monster.
The most famous 'cryptid' filmed in the UK was an unidentified animal which was captured on CCTV crossing the A338 Wessex Way in Dorset in 2007.
Haunted Yorkshire believe the figure is a humanoid-type creature galloping and leaping with the gate of canine.
The next day, they visited Sandal Castle themselves to examine the spot where the creature was seen. They estimated it to be the size of a large dog such as an Alsatian.