Good news and bad news. We can't allow this post to stay up, as the offer for Gold is a beacon for everyone to offer really bad answers, so we can't allow such offers in the title! The good news is, this did pique my interest, so I thought I'd at least make a rough try at working my way through it. But since there is no competition... unless another mod sees this... please don't feel the need to follow through with the Gold unless you really want to.

OK, 160 million people, give or take a few million, died in in war through the 20th century, at least according to this site which has added up death tolls from various conflicts.

A very high percentage of those are not battle deaths, but disease or famine caused by the conflict. In the World Wars, this was about 1/3 of all deaths, at minimum. So we can reduce the number to about 100 million now.

Of just combat deaths, there simply aren't statistics showing the cause of death with anything that one could call accuracy, but wars where tanks were a major component are probably the minority of conflicts. World War II obviously they played a major part, the Israeli-Arab conflicts... But something like the Nigerian Civil War? Small, small part. Really, tanks are going to be outnumbered in the casualties they cause by most other weapon systems. Infantry, by sheer weight of numbers, will cause more deaths, and air power, by virtue of strategic bombing, is also going to. Artillery is much more efficient as well. So lets say they all out number the tank two to one. So at ABSOLUTE best, the tank would account for 14 million casualties, and I think we still are being charitable by a very, very wide margin.

So, now we have the horse. Used by cavalry for thousands of years.

The Mongols are thought to have killed somewhere between 20,000,000 and 60,000,000 during their height, and Tamerlane is "credited" with another 10,000,000 or so to that number. Both forces were known for their horsemanship, so we can reasonably assume that a great many of those were the result of mounted warriors. Even if we assume 10 percent, that is still already more than halfway to equalling the tank. And that is only two conflicts for a period spanning thousands of years. I hope you'll excuse me for not adding the total casualties for all the wars in that time period, but I think it should be clear that even if they caused a tiny percentage of the casualties, cavalry should outnumber tanks in the deaths they have caused by a very, very wide margin. BUT, that is only really by virtue of time span. If you were to give numbers divided by time in use, or by total number of tanks vs. total number of cavalry, the tank would probably be more efficient.

So, hope that helps!