Shadow Bug

Shadow Bugs are the small, glowing, violet spheres of which the members of the Subspace Army are made in The Subspace Emissary. They are apparently embodiments of a primordial substance. Shadow Bugs were created by extracting a mysterious substance from Mr. Game & Watch, leading Tabuu to have him captured to become the source of the Subspace Army's soldiers.

They first appeared in a Smash Bros. DOJO!! update called "This World", which introduced the main plot of The Subspace Emissary, shown being dropped down from the Halberd, as well as surrounding a single Primid.

Additionally, Shadow Bugs are mentioned in for, in which Pit says, "Hey, it's a Shadow Bug!". This is a reference to how Shadow Bugs were extracted from Mr. Game & Watch.

Shadow Bug Clones
Shadow Bugs are capable of copying a character's trophy and assuming their appearance, but are also capable of altering certain qualities such as size, as seen in the fight with the giant Diddy Kong. A copy of a character created by Shadow Bugs will have a purple tinge, be surrounded by a smoky shadow aura, and possess glowing yellow eyes. Bowser himself had a platoon of Shadow Bugs which he ordered to serve as a clone of himself, both to act as a decoy and a second-in-command.

Spirit
The Shadow Bug appears in Super Smash Bros. Ultimate as an Advanced-class support spirit. It can be purchased at Beedle's Tent.

The Spirit Battle for Shadow Bug uses an army of puppet fighters and is fought on the Pokémon Stadium stage. During the battle, the stage will be covered in a poisonous cloud, referencing the Shadow Bugs' purple color and their ability to swarm.

Trivia

 * Bowser is the only character not seen as a trophy prior to having a Shadow Bug clone appear.