Are horses native species immigrant species indicator species or keystone species?

Answer 1

In North America, horses are an immigrant species. Horses were brought to the Americas by the Spanish.

Answer2:

Horses originally developed over the North American and Eurasian continents. About 10,000 years ago the horse died out in North America, it is speculated through hunting by nomadic people who immigrated to North America and possibly some kind of disease that might have swept through the equine population. Horses were reintroduced a few thousand years later by the Spanish and have since re-adapted to their native environment. This makes equines in North America uniquely both a native species and an immigrant species, but they will always remain truly native to North America.

In other parts of the world horses have almost always been native species.