|Modern horses (Equus equus) and their close
relatives (zebra, and wild asses) evolved over about 60 million years from small
forest-dwelling precursors. Several thousand years of controlled breeding have produced a
wide array of modern types, including horses built for speed, such as the Arabian horse
(upper left) and quarter horse (lower center), and for strength, such as the Scottish
Clydesdale horse (lower right). American mustangs (top center and right), which derived
from horses escaped from the early Spanish Conquistadores, became the basis of wealth among
native Americans of the Great Plains and Rocky Mountains.