Found this online which gives the numbers of horses throughout the world and breaks it down by general area including number of horses and number of horses as a proportion to the general population - the 2000 numbers would be very applicable to the original game whereas the 2008 numbers are more for the 2013 game
In 2008, there are 58.7 million horses in the world, South America dominating (15 millions) followed by Asia (13.8 millions), North America (9.8 millions), Latin America and Caribbean (8.7 millions), Europe (6.3 millions), Africa (4.5 millions) and Oceania (0.41million).
http://www.fao.org/tempref/AG/Reserv...dka_Thesis.pdf