Super Smash Bros. Ultimate has more characters than any other entry in the series, but at least one popular Pokemon was left on the cutting room floor, as an unused Poke Ball summon for Dragonite can be found in the files of the game.
One of the biggest draws of the original Super Smash Bros. on N64 was the presence of Pokemon. The game was released at a time when Pokemania was at its height, and the ability to play as Pikachu and Jigglypuff in a fighting game was a huge draw. It wasn't possible to include all of the original 151 Pokemon as characters, so some were used as temporary allies in the Poke Ball item. The Saffron City stage also had some built-in Pokemon summons as stage hazards. There are fifty-five Poke Ball summons in Super Smash Bros. Ultimate, featuring creatures from throughout the series.
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There were Pokemon planned for Super Smash Bros. Ultimate that didn't make the final cut, including Decidueye from Pokemon Sun & Moon. There is one Pokemon buried within the files of the game that was planned to be used as a Poke Ball summon, but it was never finished. According to The Cutting Room Floor, there is an unused model for Dragonite in the folder for the Poke Ball summons.
A model for Dragonite appears in the Kalos Pokemon League stage, but this unused model has a lot more detail put into its design. It's clear that it was planned to be used as a Poke Ball summon, but it's unclear what move it would have used when summoned. Super Smash Bros. Ultimate keeps receiving new content through updates, so Sakurai's team could have finished Dragonite in the two years since the release of the game, but they haven't. It's clear that there's a reason why Dragonite was never used, but we might never find out why unless Sakurai talks about it in his Famitsu column.
Dragonite is one of the most popular Pokemon in the series and it's more than worthy of receiving a Poke Ball summon in Super Smash Bros. Ultimate. Sakurai has waivered on using Pokemon content in previous games, like the planned Sprout Tower stage in Melee, or the Ditto Poke Ball summon in Melee that was eventually repurposed for Super Smash Bros. Ultimate. It's possible that a Gen 8 Pokemon is one of the remaining DLC characters for Super Smash Bros. Ultimate and it will add more Poke Ball summons to the game. If that day comes, then Dragonite might finally have a chance to terrorize the Smash Bros. roster with Outrage or Hyper Beam.
Source: The Cutting Room Floor