Equestrian creatures, like all creatures, are very important to their native ecosystems. Zebras, for example, are a main source of food for lions. The leftover parts of the zebra carcass are also food for hyenas.

Equestrian animals are also important in that they are primary consumers. They help control the growth of the various plants that they eat. The waste created by these horse-like creatures provides sustenance for many species of insect.

They help with plant growth and soil creation as well:

"Both wild horses and burros complement the North American ecosystem by dispersing the intact seeds of many native plant species in their feces, and, in fact, have been deliberately reintroduced by ecologists to fulfill this much needed ecological purpose. They also aid greatly in the creation of humus and the building of soil, as well as enhance the food chain through contributing their Earthly remains to their predators (puma, black bear) and scavengers (corvid birds, vultures, hawks, rodents, wild dogs, bobcats, insects, etc.); and even, in the final case, to the fungi which decompose their remains and to the plants which absorb their nutrients through their roots."

So, equestrian animals help plants grow that feed other animals, create soil that houses insects, and provide food for predators.

Non-native equestrian animals, however, can be damaging to the native ecosystem, as they disrupt the delicate natural balance achieved by the native species.

~ Quote from: http://www.saveourwildhorses.org/ecosystem.htm