The poem splits into two short moments. In the first, someone is alone with horses and suddenly sees "in what fear / people live." Away from other people, the speaker gets a kind of clarity: ordinary human life looks anxious and jumpy next to the calm of animals. The horses aren't described, but their presence seems to make that fear visible by contrast.
The second stanza turns inward and gets almost comic. The speaker is "mad at the dead / at Monday / even at my drink." The list slides from the huge and unanswerable (death) to the petty (a weekday, a glass), and that slide feels honest. It suggests a mood of free-floating irritation, maybe grief mixed with plain crankiness, that attaches itself to anything nearby. The word "even" hints the speaker knows it's a bit absurd.
The overall mood is lonely, dry, and a little raw. The title's cowboy image fits the stubborn, solitary stance, though the poem lets some vulnerability show through.