Published Saturday, 18 August 2012
The Northern Ireland Health and Safety Executive is investigating. (© UTV)
It happened at Eglinton Equestrian Club on Friday.
It is understood the horse fell after going over a jump, during an event which the young girl had been taking part in.
Local MLA Mark H Durkan described it as a tragedy.
"I would like to express my deepest sympathies to this young girl's family and the wider equestrian family," said the SDLP representative.
"This is an extremely difficult time for them following this tragic accident."
The incident has been referred to the Northern Ireland Health and Safety Executive.
The HSE said Derry City Council is taking the lead in investigating.