One of the final characters revealed for Super Smash Bros Ultimate’s character roster was Incineroar, the Fire/Dark-type Pokemon that acts like a professional wrestler. However, apparently only a last minute Super Smash Bros Ultimate character change allowed Incineroar to be brought in over another Pokemon that was supposed to come.
A large number of different Pokemon make their way to Super Smash Bros with every new game, whether they’re in a Poke Ball to be used for one-time attacks, or as part of the game’s actual character roster. Currently, Pokemon Trainer, Lucario, Mewtwo, and Greninja all occupy the other Pokemon slots, while Incineroar occupies the slot for Generation Seven.
However, if it weren’t for the Super Smash Bros Ultimate character change, we would have had a different type of Pokemon in that slot. According to Smash Bros director Masahiro Sakurai, the grass-type starter for the Alola Region, Decidueye, made it to the final two Pokemon being considered. However, it lost out to Incineroar.
Decidueye, being a Grass/Ghost-type Pokemon taking on a Robin Hood persona as a hooded archer, instead was put into Pokken Tournament XD. At the same time, Sakurai also wishes that he could have added more characters from other recently-released Nintendo games, such as Arms and Xenoblade Chronicles 2 (though Shulk, the protagonist from the original Xenoblade Chronicles, is part of Ultimate’s roster.)
The reasoning behind so many other characters being left out is mainly because Super Smash Bros Ultimate’s project plan had to be nailed down a long time before its release; not surprising, considering its scope. Most of the new characters that we got in Ultimate, going by that philosophy, are leftovers taken from the Super Smash Bros Wii U character polls, which asked fans what characters they wanted added (and which Bayonetta won), including King K.Rool, the Belmonts, and Ridley.
While we likely won’t see Decidueye in Super Smash Bros Ultimate, hopefully the upcoming DLC characters will all be good.