Why are the ponies in "A Blessing" referred to specifically as "Indian ponies"? The term refers to a particular breed—a small hardy horse of North America that sometimes has black and white markings (like those of the pony in the poem). This may be the only meaning the poet intended.
But as soon as the word "Indian" enters the poem, it's kind of hard to get the word out of your mind, and James Wright must have known that. So he probably wouldn't hold it against us if we explore the possibility of an historical allusion to Native Americans in this poem. Plus, a number of his other poems, such as "I Am a Sioux Brave, He Said in Minneapolis," do contain explicit references to Native American culture.
During the eighteenth and nineteenth centuries, expanding European-American populations forced Native American tribes from their homelands. In this context, "barbed wire" (7) suggests the imposition of European notions of privately-owned land, as the western prairies were carved up into private farms and ranches. The sharpness of the barbed wire also connotes pain and violence, bringing to mind armed conflicts, brutal confinement, and forced relocation of Native Americans.
In line 19, the speaker refers to the "wild" mane of the Indian pony. In this context, the word "wild" may remind you not only of the Wild West, but also of positive ideas associated with wilderness, such as the beauty of a pristine landscape. Wilderness can also imply freedom from the constraints of urban society—the kind of freedom, perhaps, that we often associate with the Native Americans' traditional way of life.
So are you buying any of this? Maybe you think these shout-outs to Native Americans are plausible. But what does any of this have to do with the situation described in the poem?
Let's not forget that this poem is called "A Blessing" (see the "Title" section for more on this). In the poem, the speaker moves from an urban world (the highway) to the more "wild" setting of the pasture, where contact with an Indian pony appears to trigger a mystical vision. Native American traditions include belief in a spiritual power that connects all things, including people, ponies, and plants. From this perspective, breaking into blossom might make perfect sense!