The Red Pony
The Tiflin ranch sits snug as a bug in a rug between two mountain ranges—the Gabilan Mountains to the east, and the "Great Ones" to the west.
We know Jody's a budding mountaineer because he names his red pony after the eastern mountains—Gabilan—because those mountains are "the grandest and prettiest things he knew" (1.45). The Gabilan Mountains "were jolly mountains, with hill ranches in their creases, and with pine trees growing on the crests. People lived there, and battles had been fought against the Mexicans on the slopes" (2.32). In other words, these mountains are settled and safe. Sure, they'd been fun to explore, and as far as we can tell, Jody's never set foot on them, but he knows what he'll find there in any case.
The Great Ones
Then there are the Great Ones. Those aren't so pleasant. In fact, they're pretty scary:
[Jody] sat up the better to look at the great mountains where they went piling back, growing darker and more savage until they finished with one jagged ridge, high up against the west. Curious secret mountains […] (2.6)
And when Jody questions Carl about them, his old man only adds to the mystery. He tells Jody that there's nothing in those mountains, but Jody begs to differ:
Jody knew something was there, something very wonderful because it wasn't known, something secret and mysterious. He could feel within himself that this was so. (2.25)
Unlike the Gabilans, these mountains aren't settled, and his father even admits that only a few people have probably ever explored them. And even those folks can't have seen it all. Jody's curious and wants some answers.
So when Gitano shows up, he peppers the old guy with questions. But no dice. Gitano doesn't remember much, and describes them only as "nice." Still, he must know a little something about those mountains, because that's just where he goes when he steals Easter. And if we may say so, Jody's a bit jealous of the man's adventure:
He looked searchingly at the towering mountains—ridge after ridge after ridge until at last there was the ocean. For a moment he thought he could see a black speck crawling up the farthest ridge. Jody thought of the rapier and of Gitano. And he thought of the great mountains. A longing caressed him and it was so sharp that he wanted to cry to get it out of his breast. (2.173)
What's this all about?
As it turns out, we don't get the answer to this question until the final part of the novella, "The Leader of the People." Jody's grandpa tells him about his journey westward with a wagon train, and then describes how the whole trip came to an abrupt end:
"There's no place to go. There's the ocean to stop you. There's a line of old men along the shore hating the ocean because it stopped them.
No place to go, Jody. Every place is taken. But that's not the worst—no, not the worst. Westering has died out of the people. Westering isn't a hunger any more. It's all done." (4.161,163)
What does westering have to do with the Great Ones, you ask? Good question. Grandfather believes that westering has died out of the people, but it certainly hasn't died out of Jody. He keeps looking west at those peaks, and wanting desperately to go explore them. So what's stopping him?
It's not the ocean. That's what stopped his Grandfather's generation. But it might as well be. What's stopping Jody is that those mountains aren't much of a mystery after all. People moved west, settled everywhere they could, and now, as Gramps puts it, "every place is taken" (1.163). Jody seems to be longing for some great old era when there were new frontiers to explore and worlds to be conquered. That's what those mountains represent. And maybe that's what's got him so bummed. Folks are done exploring. They're settled, and that means Jody will have to settle, too.