Months ago, fans of Super Smash Bros. Ultimate were wondering which new Pokémon would appear in the game, as every iteration had featured at least one new Pokémon on the roster, with Pichu and Mewtwo being added in Melee, the Pokémon Trainer and Lucario in Brawl, and Greninja in 3DS/Wii U.
The final Super Smash Bros. Ultimate Direct revealed that the Pokémon being added to the game was Incineroar, which is the fully evolved version of the Fire-type starter in Pokémon Sun & Moon.
Incineroar wasn't a totally surprising addition to the game, but some fans thought that Decidueye might have taken the spot instead, seeing as how it is a fan favorite and also appeared in Pokkén Tournament DX.
It seems that Decidueye was a lot closer to being included in Super Smash Bros. Ultimate than we realized, as Masahiro Sakurai recently revealed in an interview with Nintendo Dream magazine that Decidueye came close to stealing the spot from Incineroar. (via @PushDustin and @AllSourceGaming on Twitter)
Masahiro Sakurai also revealed that Princess Zelda's design is an amalgamation of her outfits from A Link to the Past and A Link Between Worlds, that characters from ARMS and Xenoblade Chronicles 2 might have made it into the game if they had come out sooner, and that the Belmonts, King K. Rool, and Ridley were added in due to the Smash Ballot that was held for Super Smash Bros. 3DS/Wii U.
It was also revealed that King K. Rool is meant to be a heavy character who can do things that heavies normally cannot do, which may explain why he is overpowered.
Masahiro Sakurai also mentioned that the Pirahna Plant was chosen due to the fact that it is recognizable and is also a villain, which isn't likely to soothe the wounds of the fans who are still disappointed that it was chosen over their favorite character.
If Decidueye had made it into Super Smash Bros. Ultimate, then it would likely have played in a similar manner to the three Links, as it is an archer that can sometimes use blades (in the form of Leaf Blade), which likely would have had a similar recovery to Pit or Meta Knight due to its wings.
There is still a chance we might see Decidueye as a DLC character, but we are more likely to see a representative from the next generation of Pokémon games before then.