Doctor Zone is a very old, grey horse with many age spots and wrinkles. He has a darker grey muzzle and underbelly, and drooping eyes. While three of his hooves are black, the front left one is a light grey, and he has only small sickly wires of a mane left.
In "The Wilder West", Javier tells Sara and the Doctor Zone fan club that he was referring to the horse and not the real Doctor Zone, after which they take pictures of the location thinking it was where episode 576 of the series was filmed. Afterward, he reveals that the ranch was actually the shooting location of the episode and they named the horse after Doctor Zone, to which she sings that she knew it.
“And through some of you may have doubted me, Josh, my instincts never let me down. I knew this was the very dirt where they shot Doctor Zone!” “Shot him? No sir. Doctor Zone's our favorite racehorse. We'd never shoot him! Though he is getting long in the tooth.” —Sara and Javier[source]
“Wait, wait! A racehorse named Doctor Zone. That's worth something, right?” —Sara Murphy[src]