I think a lot of it comes from ancient tribal customs. Many pastoral/nomadic groups, like the Aryans, would only eat horses as a last resort. The horse was so i

I think a lot of it comes from ancient tribal customs. Many pastoral/nomadic groups, like the Aryans, would only eat horses as a last resort. The horse was so important to the tribe's survival. Incidentally, the same taboo was probably applied to eating cows, as that animal was extremely important for Indo-Aryans' survival. Over time the original meaning of the taboo is lost, it just continues to get enforced by each subsequent generation.