The Bond Between Horse & Human
The bond between horse and boy is one of the predominant themes in the book. The Black is a completely wild horse but being marooned on an island with just Alec brought them extremely close together. Alec's love of horses also means that he is not scared of the stallion and so his relaxed demeanor calms the horse and allows them to begin building a trust in each other. The Black saves Alec's life by swimming him to shore after the shipwreck and saves his life again when he tramples a poisonous snake that is aiming for Alec. The reason they are able to win the race is that Alec has faith in the Black's ability to run without bucking him off and so he does not hold him back at all. The Black clearly trusts Alec far more than he trusts another human and Alec is, in turn, devoted to his horse.
The Spirit of Survival
After the shipwreck, Alec is totally self-reliant and could quite easily give up in the face of the enormous challenges he faces. He tries to fish and finds a type of edible seaweed to sustain himself, but the fact he is able to focus and apply himself to the task of finding food and building shelter shows that his spirit of survival is driving him forward.
The Victory of the Underdog
The Black, although an incredible physical specimen with natural raw speed, is an underdog in the horse race as his two opponents, Sun Raider and Cyclone, are very experienced champions. Similarly, Alec only learned to ride that summer in India, and has never ridden in a race; however as a team the two are able to rise above their underdog status and use their combined attributes and mutual trust to pull off an unexpected victory.
