What term refers to a wild untamed horse?

The term that refers to a wild untamed horse is a bronco. This word comes from Spanish, where it means "rough" or "untamed." Broncos are typically associated with horses that have not been broken in and can be unpredictable in behavior, making them a popular choice in rodeo activities, where they are often featured in events focused on riding and taming them.

The other terms, while related to horses, do not fit the description as precisely. A mustang specifically refers to a type of feral horse found in North America, descended from domesticated horses brought by Spanish settlers. While mustangs are often wild and untamed, the use of "bronco" emphasizes the aspect of untamed behavior more specifically.

A filly is a young female horse, and a pinto describes a horse with a particular coat coloration featuring large patches of white and another color. These terms do not convey the same idea of untamed nature.