Final answer:
A horse's emotions are best indicated through its body language and facial expressions, which can be observed in various forms of art and literature. Artistic representations, like Blue Horse I, and literary works use these physical characteristics to express emotions and create a connection between the subject and the audience.
Step-by-step explanation:
One feature that best indicates a horse's emotions is their body language, specifically their musculature and facial expressions. In the context of art, how a horse is depicted can reveal much about its intended emotions and the message the artist intends to convey. In the example of Blue Horse I, the use of color symbolism by Franz Marc to portray a horse in various shades of blue against a contrasting background, describes a sense of serenity and spirituality. Similarly, in sculptural depictions, such as the emperor's horse, the dynamism of the horse's motion and the careful modeling of its musculature suggest a powerful rendering full of life and motion. In both cases, the artists observe their subjects closely to highlight these expressive elements.
In literature, such as in the excerpt from Zane Grey's work, the description of a horse named Wrangle uses the horse's physical responses, like the trembling and vibrating of muscles, to reflect the animal's vital energy and the bond between human and horse. Artistic portrayals and literary descriptions both use detailed observation of the horse's physical characteristics to communicate emotion and expression.