Everyone knows unicorns are mythical creatures, right? Atheists certainly think so, and as a result, they mock the Bible, because the KJV happens to mention it. Obviously, if the Bible mentions a fairy tale creature, the Bible is a fairy tale.
The first problem with this line of reasoning is that there's a very good reason the KJV is the only popular translation to use this word. The re'em is an unidentified creature. There are many credible theories as to what animal it could be, such as a wild ox, the aurochs, or even a rhino, but ultimately, the identity of the re'em is lost to time.
Ironically, however, it is likely not a single horned creature. The first use of the word in the Bible (Numbers 23:22) suggests a multiple horned creature, stating that God is like the "tow’apaha" (horns, plural) of A re'em (singular). So, with all the other valid identifications for the re'em, the chances of it being a horse with a single horn are about as slim as the horn of a unicorn.
But let's ignore all of this and imagine, for a moment, the Bible does mention a horse with a horn. Is this really so unbelievable? The world is full of many interesting creatures. Horses are real. So are rhinos, cows, antelopes, giraffes, and these are just a few of the creatures that have survived. Other horned creatures, including the so-called Siberian unicorn, have gone extinct. The funniest part? Evolutionists, which most atheists tend to be, believe they all came from the same ancestor!
So why is a horse with a horn so unbelievable? Is it not possible that such a creature used to exist, yet is either extinct or rare today? It wouldn't be the first time the Bible had gone from "no" evidence to being proven right yet again. Thus, even if we were to accept that the only possible identity for the re'em is an actual unicorn as depicted in pop culture, this just wouldn't work as an argument against Christianity.
So why do so many atheists use it? Simple: They don't have much else. At this point in history, Christianity is so well attested that atheists need to pad up their case with incredulity. "Hahahaha, talking snake, pink fluffy unicorns, cosmic zombie!" Christianity is so believable, atheists need to laugh at the most unbelievable parts in order to sustain their unbelief.
But why bother sustaining that unbelief? Because we're all rebels. Atheist, Christian, we've all gone astray. No one is actually good enough to relate to God. We just keep disobeying Him. For that, we deserve His wrath. But He offers an alternative. 2,000 years ago, Jesus came and took the full wrath of God for sin. Those who repent of their sin and believe He rose from the dead will be reconciled to God. Those who don't want that reconciliation will find any excuse to reject it.