Herds of wild horses from the eastern United States were forced west by civilization and eventually crossed the Mississippi River and joined the western herds. French blood was introduced to the mix from herds pressured out of the Detroit area and from French settlers in the South in the region around New Orleans.
Another breed that probably contributed to the blood of the Mustang is the
old-type East Friesian. For a period of over 10 years during the late 1800s
and early 1900s about 150 stallions each year were purchased by the U.S.
government from Germany. The old-style East Friesian of that time was a heavy
warmblood or coach horse and was purchased to pull artillery or heavy wagons.
So wherever the US calvary was found in battles in the west these horses were
found, and undoubtedly some escaped and added their blood to that of the
American Mustang.