How does Darl refer to his mother in the section starting on page 128? and why is it different from the other chapters?

I don't think Darl sees her as a horse, but he is alluding to the fact that the horse has significant meaning to different characters. Jewel worked 24 hours a day to save enough to buy the horse, and Darl believes that he was more devoted to the horse than his own mother while she lived. But Jewel belies this when he gives up the horse so they can continue on their journey.