But this is what makes the discussion fun from the perspective of a believer. The number of places that the text mentions a Nephite riding a horse? Zero. The whole idea of someone riding a tapir has nothing to do with the text. It is simply confirms that on some level, trying to take a word that has seen lexical expansion, or that has experienced a loan-shift, and to place it narrowly back into the original context often creates absurdities. We would get the same thing if we insisted that Marco Polo really was saying that he had found unicorns - and that anyone who was trying to insist that he had really found a rhinoceros is way off in left field (let alone the expectation that Marco Polo had that unicorns could be made docile by maidens ...).
If the argument is a simplistic argument that any mention of the horse is anachronistic because there were no horses in North/South America at the time the events in the Book of Mormon were alleged to happen (and certainly no domesticated horses) it doesn't have any meaningful impact. It's not that there weren't horses. The mention of the horse isn't out of place on the Brass Plates (assuming they were real). And consequently, it's not entirely anachronistic to find horses on the gold plates (assuming that they were real also). It's all about a place and time and the contextual meaning assigned to the word horse in the Book of Mormon. And the Book of Mormon was produced in the 19th century - so it isn't anachronistic there. We only get to the idea of the horse as an anachronism to the extent that we can argue that the horse in the text is meant to represent our narrow understanding of that term when we read it - and that it must have had that intention in the ancient source (the alleged gold plates). And perhaps it is anachronistic when viewed from that narrow perspective - but - you can't make that argument by suggesting that alternative interpretations - like the suggestion that the word horse has undergone some form of lexical expansion - are simply ludicrous - especially when your way of suggesting this is to make a caricature of the text instead of discussing what the text really says.
We get the same sort of thing in the Travels of Marco Polo (only perhaps more so). If we are going to insist that Marco Polo can only mean the traditional creature unicorn when he writes about a unicorn - and that he couldn't possibly be referring to something else (like, say, a rhinoceros), then we get the same sort of caricature of the text. And it really doesn't matter (if we do this) where Marco Polo's journeys were at, because the unicorn is a fictional creature.
And so, since the believer doesn't feel the need to address any issues that are raised that don't really address their perspective, and the critic in light of the uncovered absurdity feels no need to engage the believer, everyone simply ignores the fact that there are real things that can be discussed about the text - and that there are real issues that ought to be raised in that discussion. But, we never get there - because the train gets derailed running over the Tapir ridden by the Nephite (or, the fact that "the fossil record is placing evidence of horses increasingly closer to Book of Mormon timeframes").