Megan McCulloch, 19, was riding double with a young man when she apparently lost her balance and started to fall.
"As she started falling, she grabbed the young man in front of her and, as he started falling, the saddle slipped," said Carl Mount Jr., the son of the general manager for the Lazy W Saddle Club.
"Her head struck the fence and a pole as she fell," said Lt. Mark Little of the Southaven Police Department.
The unidentified man, 21, was also injured in the accident, but Little said he refused treatment by paramedics called to the scene.
McCulloch was flown by Hospital Wing to the Regional Medical Center at Memphis, where she was listed in critical condition with massive head injuries.
DeSoto County Coroner Jeffery Pounders said the accident took place at 10:05 p.m. Friday.
McCulloch died at The Med at 4:44 p.m. Sunday.
Mount said the two people were just riding.
"They were riding sort of in preparation for a trail ride. They had ridden a different horse earlier," he said.
The quarter-horse they were on was very gentle, Mount said.
"He has a good disposition and seems to be well-tempered.
"I don't think there was anything that the horse did that caused the accident. This was just a freak accident."
Mount said the couple was not riding recklessly.
"They might have been in sort of a canter and she somehow lost her balance," he said. "There was nothing here that was intentional or anything reckless. They were just enjoying the ride and it happened.
"When the two of them fell, the horse stopped and went back to check on the riders."