In Book 6 of his War Commentaries, Caesar dabbles in ethnography, and has this description of the Hercynian Forest:
[6.25] The breadth of this Hercynian forest, which has been referred to above, is to a quick traveler, a journey of nine days. For it can not be otherwise computed, nor are they acquainted with the measures of roads. It begins at the frontiers of the Helvetii, Nemetes, and Rauraci, and extends in a right line along the river Danube to the territories of the Daci and the Anartes; it bends thence to the left in a different direction from the river, and owing to its extent touches the confines of many nations; nor is there any person belonging to this part of Germany who says that he either has gone to the extremity of that forest, though he had advanced a journey of sixty days, or has heard in what place it begins. It is certain that many kinds of wild beast are produced in it which have not been seen in other parts; of which the following are such as differ principally from other animals, and appear worthy of being committed to record.
[6.26] There is an ox 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.
[6.27] There are also [animals] which are called elks. The shape of these, and the varied color of their skins, is much like roes, but in size they surpass them a little and are destitute of horns, and have legs without joints and ligatures; nor do they lie down for the purpose of rest, nor, if they have been thrown down by any accident, can they raise or lift themselves up. Trees serve as beds to them; they lean themselves against them, and thus reclining only slightly, they take their rest; when the huntsmen have discovered from the footsteps of these animals whither they are accustomed to betake themselves, they either undermine all the trees at the roots, or cut into them so far that the upper part of the trees may appear to be left standing. When they have leant upon them, according to their habit, they knock down by their weight the unsupported trees, and fall down themselves along with them.

[6.28] There is a third kind, consisting of those animals which are called uri. These are a little below the elephant in size, and of the appearance, color, and shape of a bull. Their strength and speed are extraordinary; they spare neither man nor wild beast which they have espied. These the Germans take with much pains in pits and kill them. The young men harden themselves with this exercise, and practice themselves in this kind of hunting, and those who have slain the greatest number of them, having produced the horns in public, to serve as evidence, receive great praise. But not even when taken very young can they be rendered familiar to men and tamed. The size, shape, and appearance of their horns differ much from the horns of our oxen. These they anxiously seek after, and bind at the tips with silver, and use as cups at their most sumptuous entertainments.
Adrian Goldsworthy, in his new – and rather uneven – biography, Caesar: Life of a Colossus,
thinks that it's unlikely that a man as intelligent and educated as
Caesar would have taken such absurd tales seriously; rather that this
was a “rare note of humour,” but that it is difficult to ascertain
whether his audience would have perceived it as such. In other words,
a tremendous leg pull?