The tone of the story is simple and straightforward, but there are signs of foreboding.
Tone is the author’s attitude toward the subject. Although you could consider the tone misleading in that in the beginning you might think it is a nice story about a boy and a horse, there is plenty of foreshadowing to indicate otherwise.
Over the hillside two big black buzzards sailed low to the ground and their shadows slipped smoothly and quickly ahead of them. Some animal had died in the vicinity. Jody knew it. (ch 1, p. 5)
Jody is unsettled by the buzzards. He hates them, and calls them indecent. Therefore we have a clear indication from Steinbeck that there will be death in the story.
Steinbeck also goes out of his way to tell the story in a simple, straightforward way. Life on the ranch is not romanticized. It’s a hard life. Although Jody enjoys the pony, even its presentation to him is frill-less and ominous. Jody was ordered out to the barn, and “felt a kind of doom in the air” (ch 1, p. 8). That’s hardly the carefree story of a boy and his pony.