Let's look closely at those lines. They appear near the beginning of the story, when it's a particularly good time to pay attention to any imagery. (Why? Imagery, especially near the very start of a story, tends to set the mood, foreshadow the ending, and even give us hints about the theme.)
"A miserable horse, whose ribs were as articulate as the bars of a gridiron, stalked about a field where a thin carpet of moss, scarcely covering the ragged beds of pudding stone, tantalized and balked his hunger; and sometimes he would lean his head over the fence, look piteously at the passer by, and seem to petition deliverance from this land of famine."
So we’re looking at a very unhappy horse, one who’s so skinny that the ribs are visible. And the horse is standing in a field that’s mostly rocks with just a little bit of moss on them. He makes hungry sounds and looks sadly at people who walk by.
Some of those imagery-laden phrases are particularly telling:
1. “ribs were as articulate as the bars of a gridiron”
We’re told that the horse’s ribs are so easy to see that they look like a gridiron. (If you’re not sure what a gridiron looks like, it’s a utensil made of parallel bars; you broil food on it, such as meat.) Now we know for sure that the poor creature’s ribs poke out horribly.
2. “lean his head over the fence”
Here’s a definite image of the pitiable horse, trapped in that barren field by the fence, leaning over it to stare at people who walk by, putting his sadness and suffering and begging on display. How awful.
That’s really all the phrases we can identify that provide imagery of the horse himself, but we could also consider the imagery of the area around the horse:
3. “a thin carpet of moss, scarcely covering the ragged beds of pudding stone”
Here we’re definitely not looking at a verdant field full of abundance and greenery. No, this field is “ragged” and full of “stone,” and the only green stuff is “a thin carpet of moss.” It’s a picture of barrenness, sparsity, hardship, and hunger.
What's the point of all this imagery? By holding in mind a clear picture of Tom’s poor hungry horse, we realize the extent to which the man who owns him is cruel, greedy, and immoral. Why else would he let his horse live in such abject distress?