For good, reliable information on horses, you might want to get in touch with Johnny Carr at Banana Bank near Belmopan or the Bevises at Mountain Equestrian Trails at Mile 8 of the Pine Ridge Road. The Carrs have about 75 head and MET has quite a few, and together these folks probably have 75 years of experience with horses in Belize.
Belize books and publications by Lan Sluder:
Belize First Magazine http://www.belizefirst.com/
Fodor's Belize and Guatemala Guide
Belize First Guide to Mainland Belize
Adapter Kit: Belize
Fodor's UpClose Central America
Belize Book of Lists 2000