The horse, named Lady, crumpled to the ground in Hell's Kitchen after she had only done two rides Tuesday afternoon — stoking renewed outrage from animal rights activists.

TWU Local 100 said the carriage horse's cause of death was determined following a necropsy at Cornell University Collage of Veterinary Medicine.

The 15-year-old Standardbred cross horse died due to a small tumor in her adrenal gland that likely caused an aortic rupture, a necropsy found, a TWU representative said.