Book 3, Cows of the Rig Veda : Chapter 1, Indra's Cows Summary

With the exception of those in the cattle industries of the Americas, this emphasis on bovine powers is too easy to trivialize. Power and wealth were both intricately bound up with and often based upon cattle. For those involved with these forces directly in the current world, you will more easily understand why. For the rest, just attempt to raise them in your mind to the exalted status they truly hold, and even more held, in the culture that created these hymns. They, like money and large houses and cars and lots of land, indicate wealth.

This being the case, Indra is indicated as 'the lord of cows.' During the hymns that the table of contents labels as falling under the category of this gods name...