Symbolism, Imagery, Allegory
Lennie first overhears about companion ponies through animal-lover (and Lennie and Baily lover) Toby. Apparently, thoroughbred racehorses have companion ponies that never leave their sides. Not everyone hears about an animal that spends its life in the shadow of another creature's glory and thinks that's me, but our girl Lennie does. So who's her racehorse? Her big sister, Bailey.
Lennie tells us about companion ponies in an attempt to explain why she threw her band audition: "I'm a companion pony," she thinks, "and companion ponies don't solo" (7.35). Bailey's dream was to go to Juilliard, so Lennie feels like she can't want the same thing… even though Lennie is ridiculously talented when it comes to music. Like a good little sidekick, to refuses her own success in favor of not outshining her sister.
With Bailey gone, though, Lennie is forced to figure out how to be her own person. It doesn't come easily—we see evidence of this, perhaps, in the fact that she is so drawn toward romantic interactions after her sister's death, desperate for companionship instead of taking a good long look at herself and getting down to the business of figuring out who she is.
She can't hide forever, though, and as the story progresses, Lennie learns to stop avoiding herself and start investing in her own path. And the thing is, that even though Lennie claims to have been happy in Bailey's shadow, there's no way it's healthy to spend all of your time playing supporting cast to someone else's main character. Sarah puts it pretty well when she says:
"Being a companion pony must suck. Not metaphorically, I mean, you know, if you're a horse. Think about it. Self-sacrifice twenty-four/seven, no glory, no glamour… they should start a union, have their own Kentucky Derby." (27.94)
Yup—being a companion pony is a life lived with limitations. And like a good best friend, Sarah gets it: Self-sacrifice is no way for Lennie to live her life. When Lennie decides she wants to challenge Rachel for first chair, Sarah says, "I know. Because you're a racehorse, not some podunk pony" (30.31). As the book ends, then, we see Lennie stepping out of her sister's shadow, finally ready to stretch her legs and see how fast and far she can run. Neigh.