Jirachi was a Generation 3 Pokemon. It wasn't in RSE, so it can't be obtained anytime, including before defeating the champion. Therefore, it goes in my answer.
Rayquaza can only be found in the Sky Pillar, which is locked until after the player defeats the champion.
Every Pokemon, including these unobtainable ones, can be obtained by trading with a friend. I don't see how Huntail and Gorebyss are different.
The second-last line was a correction to your answer that lists the Regis.