system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] From where does the name "Darth Tyranous" come from? I've seen numerous people on this subreddit refer to Count Dooku: rogue jedi, sepratist leader, and Darth Sidious' apprentice; as such, but I've never heard that name coming from an official source ;

by least Palpatine self-identifies as "The Senate" in episode III. While I am looking for the Star Wars source where we first hear the name "Darth Tyranous" refer to Count Dooku, it would still be appreciated if you can find *any* source that references it, since it is hard to identify a source as the first. (For example: Knights of the Old Republic references Exar Kun, the Mandalorian Wars, and the Great Hyperspace War. But only one of those things were first mentioned in Knights of the Old Republic, but you wouldn't know which ⠀since those things are all backstory stuff). It's possible that the "Darth Tyrannus" title first appeared in Attack of the Clones (and that I just forgot it), but to confirm that I'd have to rewatch Attack of the Clones (Har, har).
gpt
He's called 'Darth Tyranus' by Palpatine when he arrives on Coruscant, at the end of Episode II.

Edit: As pointed out below, he actually calls him 'Lord Tyranus' not 'Darth Tyranus' in the scene. Doesn't change the point, but for accuracy I've added the edit!