"Food avoidances and taboos have historically been based on religion, or have functioned to demonstrate social status differences between individuals and social groupings (18). Although Leviticus is silent on the specific issue of horse (19), in 723, Pope Gregory III indicated that the eating of horses was a ?filthy and abominable custom? in his instructions to Boniface, Bishop to the Germans (20). In Ireland, the Canones hibernenses, which date from the 7th century, impose an unusually harsh penance of 4 y on bread and water for the consumption of horsemeat (20). The explanation of this nonbiblically based Canon Law is that the consumption of horsemeat was associated with pre-Christian Celtic and Teutonic religious sacrifice (20,21). The church condemnation of horsemeat consumption was directed to suppressing pagan practices and distinguishing the Christian from the heathen (20,21)."
(From The United States? prohibition of horsemeat for human consumption: Is this a good law?
Terry L. Whiting,?Can Vet J., v.48(11); Nov 2007 )
Hat tip to Vanessa Wong of Bloomberg Businessweek, who interviewed me on the subject:?What's So Bad About Horse Meat, Anyway?