Which Roman emperor is said to have planned to make his horse a consul?

Answer

Ancient writers say he planned it for his horse Incitatus, but historians doubt it was serious.

Context

The story comes from the Roman biographer Suetonius and the historian Cassius Dio, who wrote decades after Caligula's reign (37 to 41 CE). Suetonius says Caligula gave his horse Incitatus a marble stall, an ivory manger and a jeweled collar, and that he intended to make the animal a consul. Dio adds that the horse's oats were mixed with gold flakes.

Because both authors wrote long after the events and relied on hostile tradition about a ruler who was assassinated, modern historians doubt that the plan was serious, and there is no evidence that the horse ever held office. One reading, argued by the historian Aloys Winterling, is that Caligula was mocking the senators, implying that a horse could do their jobs as well as they did. Another is that the anecdote was invented or exaggerated to portray him as insane.

A consul was one of the two chief magistrates of Rome each year, a post with real power in earlier periods but largely honorary under the emperors, which makes the joke work either way. The careful statement is that ancient writers say he planned it, not that he did it. Claims that he had a mental illness are likewise speculative, since there is no reliable diagnosis from sources of this kind.