why does the poet sey that the horse must 'think'?

You mean from "Stopping by Woods...," I assume. In the poem, the poet says "my little horse must think it queer" that he stops to look at the house in the dark. There are a few things this could mean. One, it could just be a rhetorical way to suggest that it is a strange thing to do when on one's way home at night, to waste time. Two, a horse's function is to move, to get the rider home, so the horse represents the rider's desire to make it home - so it is strange (queer) to waste time by stopping.