Ruben Flores of Peotone, Ill., said his 3-year-old quarter horse Talented Magician was found with a large lump on the left side of his neck last week, the Joliet (Ill.) Herald-News reported Tuesday.
"Someone apparently attempted to inject an unknown substance into the horse's carotid artery [and] missed," a spokesman for Will County Sheriff's Police said.
Flores worries the substance may have been a steroid, which would disqualify him from running for a $100,000 purse next month at Hialeah Race Track near Miami.
Breeder's Cup data shows Talented Magician has one win in nine starts but has finished at least third in his last three races.
Police said they had no information on possible suspects in the case.