It sounds a little odd, but it is important to know that koi and goldfish will cross if kept in the same pond.
These koi-goldfish hybrids will grow to be bigger than a goldfish but smaller than a koi. Although studies have revealed that the hybrids are anatomically intact, they are sterile. Therefore, you can only get koi-goldfish hybrids by mating koi and goldfish; the hybrids themselves will not produce offspring. Also, the hybrids will not have barbels.
An important aspect to note about keeping koi and goldfish together is that you will likely end up with baby goldfish and baby hybrids, but no baby koi. While koi will not eat goldfish eggs, the goldfish will eat the koi eggs; neither appears to eat the hybrids. Many times pond owners will think they have a lot of baby koi, but in reality they are just baby goldfish and hybrids.
Some people like hybrids and some don’t; it’s really a personal preference. It is important to remember, though, that the possibility does exist when keeping koi and goldfish together in the same pond.