The word "appendix" is from a Latin word, first used in 1500's, meaning "to append" or to add on. In books and so forth, the Appendix is something we're familiar with. In AQHA registry, the term "appendix" refers to a special registry "added to" but separate from the AQHA registry. This is excerpted from the AQHA web site at this link: Appendix vs. Quarter Horse "AQHA has created a special registry and numbering system for these Quarter Horse-Thoroughbred crosses. These horses are considered Quarter Horses, but are named for the registry system created for them — Appendix. They are denoted by a registration certificate that is white with a gold border (or completely gold if you have an older-style certificate) instead of blue. Their registration numbers also begin with an X. AQHA has registered more than 600,000 Appendix horses to date."
This was a great question! I had always wondered that, too, so I looked up the AQHA info....there's more at the link I inserted above.