Holy Cow...or Horse?

We were in Mexico in January and Ale was picking up Spanish quickly after a few days of being there. He learned that a horse was a "caballo" (ka-buy-yo...more or less). We had a chuckle when we passed a heard of cows that were grazing in a field near the highway and he said, "Mira (look), los caballos". We told him that cows were "vacas" but he wasn't concerned. He kept on calling cows "caballos" and we couldn't figure out why. One day he pronounced it and we finally picked up why....he was thinking and saying "COW-ballo". At least that explained it!