Missouri designated the Missouri mule as the official state animal in 1995. Mules were introduced to Missouri in the 1820s and quickly became popular with farmers and settlers because of their hardy nature.

Missouri mules pulled pioneer wagons in the 19th century and played an important role in moving troops and supplies in World Wars I and II. For decades Missouri was the nation's primary mule producer.

Mules are hybrids, the offspring of a mare (female horse) and a jack (male donkey). The reverse (the offspring of a male horse and a female donkey) is called a hinny. Mules and hinnies are almost always sterile because the two species have a different number of chromosomes (donkeys have 62 chromosomes, horses have 64).

Source:

State Symbols of Missouri - State Animal: Missouri SOS
Mule: Wikipedia.org

Links:

State Animals - Mammals: 50 States List