The red deer had to be put down after being attacked by Franck Hiribarne's young Irish Setter puppy and then hit by a car as it tried to run away.
Video captured by a cyclist shows the moment bystanders rushed to chased the dog away from the injured deer as it continued to circle the stricken animal.
Owner Mr Hiribarne, 44, a marketing businessman, told magistrates he was training his young puppy in Richmond Park, south west London, in October last year when the pet started chasing a deer.
The dog had been allowed to run loose - despite signs telling visitors to put their dogs on leads, the court heard.
