This model was released in the Series III Spanish Breeds Set #3960, Winners Choice Micro Horses by Creata International in 1998. Breyer bought this mold in 2004. This set was considered a Special Run or Collector's Edition for Specialty Market stores (smaller retail stores). The Series III models had a very short sales run as there were production issues with Creata International.
Sidenote: These models are referred to as Series III or 3 even though the pictured brochure lists them as Series 1. I'm not sure if this was a typo, or if the supplier, The Horse Collector Ltd, listed them that way for their own purposes.