Who is Johnny Tivio from The Man Who Listens to Horses and what is their importance?

Johnny Tivio has the honor of being Monty's all time favorite horse. The stallion weighs about 1200 pounds and has a copper coat. He is one of the best horses on the circuit for years and is undefeated in the stallion sweepstakes..... a pentathlon for horses. Monty learns a lot from Tivio and is devastated when he dies at the age of 25.