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.

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.

On Tuesday, a carriage horse in Manhattan collapsed and died on the way to its stable, according to a local union. Authorities learned of the incident around 2:30 p.m. after receiving a 911 call requesting assistance at the corner of West 51st Street and 11th Avenue in midtown.