This page is a list of horse an pony breeds, and also includes terms used to describe types of horses that are not breeds but are commonly mistaken for breeds. A breed is defined generally as a viable true-breeding population, and its members are called "purebreds." In most cases, bloodlines are recorded with a breed registry. However, in horses, the concept is somewhat flexible, as open stud books are created for fairly new types of horses that are not yet fully true-breeding. There are also a number of "color breed", sport horse, and gaited horse registries for horses with various phenotypes or other traits, which admit any animal fitting a given set of physical characteristics, even if there is minimal or no evidence of the trait being a true-breeding characteristic.