Part of the fun of reading ancient history lies in understanding that, sometimes, whether the events one is reading about actually happened is secondary to the interest generated by the fact that someone, for some reason, decided to write this stuff down in the first place.
Ancient history should not necessarily always be taken as a reliable account of the events it purports to describe but rather as a way of discerning the attitudes, intents, and mindsets of the authors who describe them. Sometimes, for example, there might be little 'historical' value in works such as Historia Augusta, with its lurid and clearly fantastical tales of the lives of the emperors, but what remains of interest is the intent of whoever wrote this gibberish down. Clearly, he (and it was nearly always a 'he') is trying to communicate something to someone for some reason, even if it is just a rather silly joke.
Another thing that soon becomes apparent with ancient history is that the 'someone' in question is clearly not you or I. Much as modern humans like to think that they are the central characters in the role-playing game of life or that we are at the peak of evolutionary biology, we are not the audience these ancient authors had in mind when they were compiling their works. Understanding who the intended audience actually was goes a long way to understanding the motives behind the author's works.
Most of the time, those ancient authors were elite men, and the audience they had in mind was their own peers. As such, the sources are mostly written with the understanding that the audience is already aware of a lot of unwritten background information and context and, importantly, that they are already in on a lot of the jokes, nudges and metaphorical sly winks. Part of a modern historian's job is unravelling that context for our own audiences, hence the glorious safety net of the endnote or footnote. Modern historical analysis of ancient sources is basically one long footnote.
So, when someone like Suetonius starts telling us tales of how, say, Julius Caesar was the 'sodomite' of the Bythinian king Nicomedes, he is not necessarily accusing him of being in a homosexual relationship and more of taking the submissive role in a diplomatic arrangement. Of being Nicomedes' 'bitch' if you would prefer. Unspoken in Suetonius' words is the understanding that throwing around such slander was a relatively common way of taking digs at political rivals and, as such, were not supposed to be taken as especially barbed attacks and more as a generalised mud-slinging. As such, they are also rather easy bullets to dodge, especially when they are fired with such random abandon. Consequently, of course, one might then pay slightly more attention to more carefully aimed public slander, such as when Pompey calls Caesar 'Aegisthus' (Suet. Jul. 50) after the Greek legend of Aegisthus, the son of Thyestes, who slept with his own daughter. Does Pompey know more than he is letting on? To us, not the intended audience, this might seem like an interesting aside. To Pompey's (and Suetonius') audience, this is a shocking slur and one they would immediately have recognised.
In this way, when Suetonius tells us (Suet. Cal. 55) that Caligula intended to make his horse, a fine beast named Incitatus, 'a consul', it should be understood that this is less an account of a slightly bonkers emperor trying to make his favourite pet into one of the three people who (technically at least) ran the Roman Empire and more a nudge-nudge that tells the audience 'Look how bonkers this bonkers bloke is.' Nobody in Suetonius' audience would be expected to believe that Caligula actually intended to make Incitatus a consul, and none of them would have thought for a second that Caligula actually could make Incitatus a consul.
None of this stops more modern audiences from believing that Caligula made his horse 'a senator' because they do not have the same context as Suetonius' audience, and neither should they be expected to.
So, did Caligula make his horse a senator? In a word, no. The context here is that, like many of the Julio-Claudians, the relationship between the emperor and the senate was, to put it lightly, rather strained, particularly as the senate did, to put it concisely, absolutely nothing but get on the emperor's nerves. In threatening to make Incitatus a 'consul' (rather than 'just' a senator), Caligula is saying, 'My horse could do a better job than you lot.'
The myth about Incitatus then grows some rather bizarre legs, including that Caligula and the horse were rather more than just friends - if you get my drift. The sources make no such suggestion, and the origin of this spin-off from the myth appears to be Tinto Brass' rather excitable and infamous 1979 movie, Caligula, with Malcolm McDowell as a dead-ringer for the titular bonkers chap.
Caligula itself has spawned all sorts of fantastical myths, and, to be honest, it's not that bad a movie. A troubled one and a total mess, sure, especially for one written by Gore Vidal and featuring such luminaries as Helen Mirren, Peter O'Toole and John Gielgud, but not as bad as the rumours would have it. The principal problem with the movie, and the source of all the salacious myths about it, was that a large part of the funding came from renowned pornographer Bob Guccione, who wasn't about to pass up the opportunity that a film set filled with partly clad extras and fake marble columns presented to him and so, famously, brought in his own film crew in the evenings to shoot his own, rather more 'adventurous' movie. I haven't seen that version yet, but it would appear that in the blurring of the lines between the Tinto Brass version and the Bob Guccione one, the lines between the 'real' myths about Caligula and the 'fake' ones are equally smudged.
Everyone is allowed to bandy about their own myths, of course; ancient history doesn't have a monopoly on such things, after all. It's just that each set of myths needs context, or people start to believe all manner of gibberish.
So no, Caligula didn't make his horse a senator and neither did he sleep with it.
He might have slept with his sister, though. That's one of those particularly well-aimed ancient barbs that seem to be aimed with some precision.
As for Incitatus, it was normal for emperors to have a favourite horse - even Napoleon had a favourite, Marengo - and Caligula is no exception in this respect. Hadrian's favourite horse was called Borysthenes and is memorialised with a fine tomb on the outskirts of Nimes, France.
Incitatus led a pampered life in a stable attended by servants who fed him oats mixed with gold flake. Cassius Dio also claims that Caligula made the animal a priest (Dio, LIX 28). Suetonius claims that he lived in his own 'house' where the emperor would invite dignitaries to dine with him, that he slept in an ivory manger, wore purple blankets (mocking the purple stripe of the togas worn by senators) and wore a collar made from precious stones. Caligula would pour libations to Incitatus' salus (health and well-being) during religious ceremonies. Caligula was, rather famously, fond of 'humorous quips.'
None of this is true, of course, and neither was it supposed to be taken as true. Both Suetonius and Dio wrote a long time after the events, and their intentions are two-fold. Firstly, by painting the Juilo-Claudians in a bad light, they can make the reign of the emperors they are living under (Hadrian for Suetonius, say) seem much more rational and benign and, secondly and perhaps more importantly, both of them knew who their audience was and what they liked.
All authors write with an audience in mind, even when they write 'from the heart' and, for example, I am no different. I'm writing this article partly because someone asked me the question and partly because I think you, my reader, will enjoy it. If I didn't think you'd enjoy it, then I would write something else as, at the end of the day, this is all about you, not me. As a result, I throw in little morsels that I think you are going to enjoy and, like the coquettish tease that I am, sometimes they are rather lurid. I know what you like, you saucy minx!
Suetonius and Dio are no different, and whilst their intended audience was never you or I, they also knew that their audience loved a bit of salacious tittle-tattle. Sating that particular thirst with some nudges and winks about mad emperors and horses worked just as well 2,000 years ago as it does today.
References and Further Reading
Cassius Dio. (1914). Roman History (E. Cary, Trans.). Harvard University Press.
Suetonius. (2007). The Twelve Caesars (R. Graves, Trans.). Penguin Classics.
I believe that Catherine the Great was also rumoured to have had a favourite horse, although its name is lost to history.
I am still waiting for a present-day 80s-tribute band called, "Ronnie James Cassius Dio."