There are several lineages of koi that are accepted throughout the world. The categories are based upon the koi's markings, colors, and/or scale type.
Many Beautiful Koi Varieties For Your Pond
15 Most RARE and BEAUTIFUL KOI Fish Varieties
The Koi, Fish that Brings Fortune - Taman Safari Bali
The Making of a Champion: How to Grow Large Beautiful Koi, by Kodama Koi Farm
40 Koi Fish Facts You Have To Know About This Magical Carp
30+ Beautiful Koi Pond Ideas for Your Garden
How to Prepare Your Pond for Quality Koi
11 Pond Landscaping Ideas You'll Want For Your Maryland Yard
A Beautiful Japanese KOI Pond Hintergarten, Zen-garten, Japanischer garten
Koi 101: What You Need to Know - Phoenix Home & Garden
Koi Pond Care for the Summer - Nualgi Ponds
What Kinds of Fish Can I Put in My Pond? - Splash Supply Company
Doitsu Variety - The beauty of no scales