By Julius Caesar (with Gordon Rugg)
First, the bad news: This is Caesar writing on the subject of the unicorn, rather than Caesar riding on a unicorn.
XXVI.–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.
(From De Bello Gallico.)
Quite what he was describing, if anything, is anyone’s guess.
What with that and his stories about elk hunting, you start to wonder how much trust you can put in his accounts of subduing the Gauls…
https://hydeandrugg.wordpress.com/2014/05/02/friday-funny-elk-hunting-101/
So that was Caesar on the unicorn. Not quite as exciting as you might have hoped.
By way of consolation, here’s Theodore Roosevelt on the moose. As far as I know, it’s real, not Photoshopped; it presumably relates to his connection with the Bull Moose Party, in American politics.
I hope this image lives up to your wildest expectations, and brightens your day.
http://www.pinterest.com/pin/356347389235275615/
Notes
The Caesar quote is from Caesar, C.J. De Bello Gallico. Project Gutenberg; Everyman’s Library version, 1915 edition, translated by W.A. MacDevitt.
I’ve corrected one minor typo in the original.
I’m using the Roosevelt photo under “fair use” terms, since it’s being used here non-commercially, and is a low-resolution copy of an image that has already been widely circulated on the Internet.
hyde and rugg 
This seems to be a garbled description of the reindeer, which has of course two antlers, but the general description of the shape is correct, and it’s the only deer where both sexes have antlers.
There are a couple of problems with the reindeer as a candidate. The reindeer’s antlers don’t form a shape like a palm (I presume he means that the shape is like the antlers of a fallow deer or a moose, with a broad flat area). Also, reindeer are from much further north than the area he’s describing. As you say, it may be a garbled description that includes some features of reindeer, and some features of something else. The bit about the single horn is also odd.
“The reindeer’s antlers don’t form a shape like a palm”
Not always, but in some cases they do, for example here:
Of course he probably heard it from people who never saw one themselves and for whom it probably was an exotic half-legendary animal from the far north.
Fascinating; thanks!
I’ll be blogging again about unusual animals over the next few weeks, including Herodotus on flying snakes.
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