Marie-Thérèse de Bourbon was a human woman who is now a ghost.
This wealthy French noblewoman and socialite was a friendly acquaintance of the Ravenswood clan prior to their arrival in Thunder Mesa; she later came to the family's financial aid after the 1860 earthquake, when Arthur Ravenswood's finances were left in a bad state by the catastrophe compounding the lavish spending of the late Henry Ravenswood, Arthur's brother. Marie-Thérèse spent the rest of her life in Thunder Mesa and was eventually buried in Boot Hill Cemetery, where it is said a supernatural heartbeat can still be heard from her coffin.
Behind the scenes
Marie-Thérèse de Bourbon was created for the backstory of the Disneyland Paris ride Phantom Manor, which opened in 1992. Her tombstone and its heartbeat were supposed to be visible in the "Boot Hill Cemetery" queue. However, she was cut to simplify the ride's storyline, and thus her existence wasn't revealed until the 2012 short story Family Secrets. The coffin with the heartbeat does exist in the finished ride, but is instead that of Melanie Ravenswood herself.
There existed a historical Marie-Thérèse de Bourbon, better known as Marie Thérèse de France; the daughter of King Louis XVI (the last King of France before the 1789 Revolution) and his wife King Marie-Antoinette, she lived in exile in Great-Britain for a significant period of her life. However, unlike the fictional Marie-Thérèse de Bourbon, she eventually settled in Austria, where she died in 1851.
It is unclear if the fictional character is a uchronical version of this Marie-Thérèse who chose the Americas as her final place of exile and lived a decade longer, or simply named for her without necessarily being her. At any rate, it is unavoidable that "Bourbon" is the family name of the French royal family.