Incitatus

Incitatus was Emperor Caligula's favorite horse. He lived in Ancient Rome around 40 CE. His name means "fast-moving" in Latin.
There is a story that Caligula made his horse a senator, but it is probably not true. According to historian Mary Beard, no ancient historian who was alive at the same time as Caligula ever said Caligula really made Incitatus a senator, and it is likely that he only told a joke about doing it.
Ancient historians do say Caligula gave his horse many gifts. In 121 c.e., in his book "The Twelve Caesars", the historian Suetonius says of Incitatus: "Besides a stall of marble, a manger of ivory, purple blankets and a collar of precious stones, he even gave this horse a house, a troop of slaves and furniture, for the more elegant entertainment of the guests invited in his name; and it is also said that he planned to make him consul."
In fiction.
In Robert Graves' 1934 book "I, Claudius", the fictional Caligula does make the fictional Incitatus a senator and gives him an ivory trough to eat out of. This is one of the ways Graves shows that Caligula has become mentally unstable. After Caligula dies, Claudius treats Incitatus like a normal horse again.
A talking version of Incitatus appears as a villain in Rick Riordan's 2018 young adult novel "The Burning Maze".