Gobblin' Apples All Day
Goblin is Johnny's horse, and he's significant to the novel because of the many ways he and Johnny are alike. He's like the animal version of Johnny—just let us count the ways.
First, Johnny and Goblin are both from well-known and highly respected families, but neither of them has any relationship with those families. Johnny has the whole Lyte situation going on, and Goblin can't live up to his sire's reputation, which is how Rab got him cheap. Rab says:
"We could no more afford to own one of Yankee Hero's sons than we could the Lyte's coach unless there was some little thing wrong with him." (5.2.9)
There's also something wrong with the otherwise brilliant silversmith—namely, a deformed hand—and together, Johnny and Goblin work on healing each other. Johnny loves Goblin, and Goblin lets himself be loved. Also, both of them enjoy attention: "Even if they had dawdled a bit on country roads, they both liked arriving at the inns at a gallop" (5.2.25). Finally, they both tend to react quickly and without thinking and need to get that part of their personalities under control.
When Rab says, "Now what you've got to do is get his confidence so completely he'll know you'll never let anything hurt him—you can't do that by whipping him. Then he'll go through Hell, a laundry yard from his point of view, for you" (5.2.4), he might as well be talking about Johnny. Look at what Johnny will do for people he trusts and cares about—he throws himself into revolutionary activities, helps Cilla whenever he can (after he realizes she's pretty amazing), and puts himself at great personal risk to find Rab after the war starts.