Horses are very helpful for farming because they can pull things like plows to give a food surplus to americans.
Horses impacted dieases because they gave people dieases like salmonellosis, ringworm, anthrax, and brucellosis. Although These dieases were deadly they did not have a huge impact on the americans.
This is why horses are the BEST animals, and were the most useful thing in the conquest of North America.