According to Julius Caesar, in the Hercynian Forest on the far side of the Rhine River,
There is an ox [or “quadruped”] of the shape of a stag, between whose ears a horn rises from the middle of the forehead, higher and straighter than those horns which are known to us. From the top of this, branches, like palms, stretch out a considerable distance. The shape of the female and of the male is the, same; the appearance and the size of the horns is the same.
Karl Shuker wonders if the creature described is a mutant deer. Beachcombing suggests it may have been a reindeer, perhaps either in profile or after one of its horns had fallen off. (They do that, you know.) Either way, the Hercynian unicorn is an intriguing ancient cryptid.