It can be argued that all recognised breeds of horse were initially crossbred in order to establish the breed independently; however some crosses are recognised and others are not. Generally speaking, if the differing parentage of the horse is known then the term crossbreed can apply. Sometimes the parentage is not known, and in this case the horse might be referred to as a ‘grade horse.’
Some crossbreeds are recognised as breeds in their own right, an example being the Anglo-Arab which is a cross between a Thoroughbred and an Arabian. Because the breed is well conformed and boasts the best qualities of its differing forebears, it is recognised independently. Most of the Warmblood horses that are recognised as breeds are in fact crosses between Thoroughbreds and draught horses.
While it is usually quite easy to tell which breed(s) has contributed to the bloodline of a horse from its appearance and conformation alone, the bloodline of some horses is so heavily influenced by differing breeds that the ancestry is unknown. Equines with unknown ancestry or ancestry that is known but not recognised, come under the category for ‘crossbreeds.’