Symbolism, Imagery, Allegory
Rain goes both ways. Some folks would give their left arm for just a drop (like the Joads in that other Steinbeck book). Some folks would give their right for the rain to cut it out. So which camp do the Tiflins fall in?
The first one. In The Red Pony, rain is nothing but bad news. When Jody gets Gabilan as a gift, he's pumped. That is, until it starts pouring for days on end, and he can't do anything but keep the poor pony inside. In this case, rain = big bummer.
And it only gets worse. When the rain lets up for a bit, and they decide to let Gabilan out for a spell, and Billy promises Jody, "If it comes on to rain—why a little rain don't hurt a horse" (1.97). Oh Billy. Why must you say things?
Of course just a few lines later, he says it again: "A little rain never hurt anything" (1.112). Um, Billy? It totally does. When the rain comes, it changes everything, and it definitely hurts the Tiflins. Okay maybe just Jody. Because when his pony dies from exposure to the elements, the kid is clearly crushed.