The horse's handler was sitting in the buggy near Pier 45 when the horse "seemed to spook for no reason" and flung the handler to the ground at about 6:30 p.m., San Francisco police Sgt. Lyn Tomioka said.
The handler suffered life-threatening injuries and was transported to San Francisco General Hospital, according to Tomioka.
The horse ran loose for about 20 minutes, during which time it knocked a woman off her bicycle, before the animal was captured near Pier 27, Tomioka said.
San Francisco Animal Care & Control officials responded and determined there was no mistreatment of the horse, according to Animal Care & Control Capt. Vicky Guldbech.
"The horse was just spooked," Guldbech said. "There wasn't anything unusual, there was no disturbance or neglect or abuse. The horse just had a bad night."
The horse was treated by a private veterinarian and returned to its stables in Petaluma, according to Guldbech.