From the publisher's website:
The Genius Guide to Races of Hoof and Horn added three new playable races to the Pathfinder RPG rules: the asterion, lapith, and piper. Since that PDF was released, Paizo has released the Advanced Players Guide, which contains new options for races to have alternate racial powers or alternate favored class options. In an effort to keep our three new races up to date, this short, free supplement expands their options to include their own set of alternate racial traits and favored class options.
Since many of our readers use like to use material from a variety of sources, along with options drawn from the Pathfinder Core Rulebook, the alternate favored class options presented here include base classes that have been published in the Advanced Player’s Guide and the Genius Guide line of products. For your convenience, we have noted the source of options that are not in the core rulebook.