Andrew Rodgerson, 26, warned a stud manager not to run 2008 St Leger winner Conduit in a valuable Group One race at Ascot after he forgot to place an accumulator bet on behalf of a syndicate.
He panicked when he realised that victory for Conduit in last July's King George VI and Queen Elizabeth Stakes would lead to the syndicate expecting winnings of 50,000, Bolton Crown Court heard.
Rodgerson, of Balderstone, near Rochdale, sent a series of text messages and emails to Peter Reynolds, the general manager for the Ballymacoll Stud Farm in Ireland which owned Conduit.
Ten days before the race he texted Mr Reynolds: "Dear Peter, we would just like to warn you should Conduit run in the King George then the horse will be killed."
Five days later he followed up with an email to the general manager, which read: "Dear Peter, I don't believe you are taking the threat of death to Conduit very seriously. We want the horse removed from the King George this weekend. If you co-operate the horse will live."
Police in Newmarket were informed and Rodgerson was arrested at his home address two days before the King George. He admitted making the threats. Rodgerson pleaded guilty to threatening to commit damage at an earlier hearing after an initial charge of blackmail was dropped.
Sentencing him to 34 weeks in jail, suspended for two years, Judge Angela Nield said he had embarked on a "foolish escapade". She accepted his actions had no practical consequences in that Conduit lined up for the King George and went on to win, but she said a message of deterrence had to go out.