With this statement Nehemiah paints in the mind of his readers a picture very much in step with the secrecy he had earlier intimated. Had all his men ridden an animal, their departure from the city would have been nosier and hence more noticeable. That Nehemiah himself rode an animal (presumably a donkey) might perhaps be attributed to the sort of protected and non-physical lifestyle to which he was hitherto accustomed. The animal will receive mention in Nehemiah 2:14 to give a sense of how tumbled and impassable the stones of the wall had become.
12 Then I arose in the night, I and a few men with me. And I told no one what my God had put into my heart to do for Jerusalem. There was no animal with me but the one on which I rode.