How do we first learn that a horse (not a human being is telling the story?

In the first three paragraphs, we are easily able to infer that the speaker in an animal, who lives in the meadow. In the fourth paragraph, the speaker alludes to himself as a colt.

There were six young colts in the meadow besides me; they were older than I was; some were nearly as large as grown-up horses. I used to run with them, and had great fun; we used to gallop all together round and round the field as hard as we could go. Sometimes we had rather rough play, for they would frequently bite and kick as well as gallop.