What important event took place in land plant evolution in the Miocene Epoch that influenced the evolution of the ungulate mammals, such as the horses?

Answer: During the Miocene age, the earth experienced a globally warm temperature and due to this, the plant varieties undergone evolution, supporting the advancement of grassland ecosystem and forests also started expanding, leading to a re-diversification of that existing ecosystem in terms of temperature.

Due to this evolutionary changes in plants, the animals like horse experienced changes in the morphological features. It is because of these, the evolution of this organisms took place.