Koi fish are really a type of carp. The term "mongrel koi" refers to koi fish which have somehow become inbred with other types of fish. Koi fish are popular for their colors and sleek appearance in fish ponds.
Koi can be found in koi ponds as a decorative fish used to adorn gardens and outdoor spaces. There is not truly a designation of a mongrel koi; the term is used in jest to refer to the undetermined lineage of the koi fish generally. The only real use of koi is as a display fish in a koi pond. Koi ponds are typically home to multiple fish and are valued for their appearance. Even though a koi commands a higher price than most carp varieties, their worth lies solely in their sleek appearance and attractiveness while swimming in fish ponds.