We step over the barbed wire in to the pasture (7)
The barbed wire fence in the poem separates the world of nature (the ponies' home) from the artificial, man-made world of the highway. Isolated from nature, the speaker must be willing to cross this boundary, taking the first step to bridge the gap.
Where they have been grazing all day, alone. (8)
The word "alone" suggests that the ponies, too, suffer from isolation. They are at home in the world of nature, and they provide companionship for each other, but they are separated from the humans. So the desire for connection seems to flow both ways.
There is no loneliness like theirs. (12)
The word "loneliness" conveys the emotional consequences of isolation. What kind of loneliness do the ponies experience, and why is it unlike other kinds of loneliness? The speaker does not explain, forcing us, as readers, to explore our own experience of loneliness in an effort to understand the ponies' loneliness.
I would like to hold the slenderer one in my arms, (15)
Who wouldn't want to hug that beautiful little black-and-white pony with the mane falling wild across her forehead? Yet the speaker doesn't actually follow through on this impulse ("would like"). Something is holding the speaker back. Is it fear of spooking the pony? Or fear of letting go emotionally? For whatever reason, the speaker does not take this step to bridge the separation from the world of nature.
Suddenly I realize That if I stepped out of my body I would break (23)
Notice the conditional tense of this statement ("if"). Again, something is holding the speaker back, and there's a hint of fear in the words "I would break." For the speaker, contact with the ponies seems to have triggered awareness of a spiritual dimension of nature, a unifying energy that transcends the physical world. But the speaker is not yet ready to step "out of my body," to accept the reality of a spiritual dimension that would end the speaker's isolation once and for all.