Barbaro was an established Dirt Horse at the time of the Derby.

Animal Kingdom won only one Dirt Race in his entire Career. It just happened to be on the big day. A winning wager on Animal Kingdom, was more of and odds and opportunity bet than a shrewdly placed tactical wager. He did run second in the Preakness, to the horse many factored would beat him.

Helium looks a little more promising as far as a Backable Horse, but a breakdown of his races doesnt suggest something remarkable.