Wasn’t Caligula the first emperor to make the Roman Senate declare him to be a god?
He also made his horse a god.
From the unreliable Wikipedia:
In AD 40, Caligula began appearing in public dressed as various gods and demigods such as Hercules, Mercury, Venus and Apollo. Reportedly, he began referring to himself as a god when meeting with politicians and he was referred to as Jupiter on occasion in public documents.
And this:
Caligula had the heads removed from various statues of gods and replaced with his own in various temples. It is said that he wished to be worshipped as Neos Helios, the New Sun. Indeed, he was represented as a sun god on Egyptian coins.
And this:
Caligula's religious policy was a departure from that of his predecessors. According to Cassius Dio, living Emperors could be worshipped as divine in the east and dead Emperors could be worshipped as divine in Rome. Augustus had the public worship his spirit on occasion, but Dio describes this as an extreme act that emperors generally shied away from. Caligula took things a step further and had those in Rome, including Senators, worship him as a physical living god.