Is CBD legal for horses in USA?

Yes, CBD is legal to purchase in all 50 states. In fact, in 2018 the U.S. Government ruled that the hemp based CBD (with less than 0.03% THC) is legal to sell and consume in any state. It is for this reason that CBD has become a very popular product in the last few years.

The FDA does not regulate it so it behooves you to choose a reputable company that is willing to disclose the source, extraction method, and analysis of their products.