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.