The sport was already reeling from a series of doping suspensions and breakdowns before seven horses died in the build-up and during the showpiece event.
And on Sunday Rio Moon, trained by Dale Romans, had to be euthanized after breaking his leg.
The three-year-old colt sustained the injury in the sixth race at the Louisville track on Sunday.
Dailymail.com has reached out to Churchill Downs for comment.
An eighth horse has died (not pictured) at Churchill Downs in just over two weeks
PETA Senior Vice President Kathy Guillermo said in a statement to Dailymail.com: ‘The carnage continues with an eighth dead horse at Churchill Downs, another fatality at Belmont Park, and an injured horse being taken away in the equine ambulance at Pimlico.
‘Horses aren’t safe at any of these ‘Cripple Crown’ tracks—or at any other horse racing facility. Get the deaths down to zero or bury this ‘sport.”
The animal rights group also said ‘enough is enough’ on social media.
Rio Moon ‘suffered a catastrophic injury to his left foreleg a few strides after the wire,’ according to notes in the Equibase chart for the sixth race.
Two horses were euthanized after being injured in races on the Kentucky Derby undercard on May 6.
Mage (No. 8) won Kentucky Derby, beating Two Phil’s and Angel Of Empire last Saturday
Saffie Joseph Jr. was suspended by Churchill Downs ahead of the Kentucky Derby after the shocking deaths of two of his horses within a 71-hour span of each other
Three-year-old gelding Chloe’s Dream got hurt in the second race of last Saturday’s meet, while Freezing Point, a three-year-old colt, was euthanized, after pulling up in the Pat Day Mile Stakes.
Five others were euthanized after training and racing incidents at the track in the days leading up to the Derby.
Two of the horses were trained by Saffie Joseph Jr. He was indefinitely suspended by the track, although investigators have yet to determine a cause for the deaths of his horses.
Mage won the 149th Kentucky Derby and will run in the Preakness on May 20.