The old interpretation of horse evolution is wrong since we now have more fossil data showing that many lines of horses evolved, most of which are now extinct. The evolution of horses was not a simple movement from one species to another to another. At many times, there were a range of horse species which coexisted in time but in different habitats.
Work Step by Step
Compare the old interpretation as described in the question with the data shown in Fig. 21.12. Especially note the branching tree and how it differs from the linear series of species described in the old interpretation. Further, see the text about coexistence of multiple species in past times as well as the time scale on Fig. 21.12 and the extent of the lines which graphically shows multiple species extant at the same time (for example, draw a horizontal line from 15 mya across. All vertical lines which this intersects represent species alive at the same time.