As in every Smash Bros., Poké Balls release a Pokémon that helps whoever threw them. This entry adds the Master Ball, which almost always releases a Legendary or Mythical Pokémon. Here are the Pokémon, according to Poképédia.

The Pokémon of Poké Balls

PokémonMoveEffect
MeowthPay DayPelts the nearest opponent with coins
ElectrodeExplosionExplodes after a delay; it can be thrown again before then
GoldeenSplashSplashes… to no effect
StaryuSwiftFires stars at the enemy
EeveeTackleLeaps from left to right
SnorlaxBody SlamGrows and lands on a wide area
TogepiMetronomeA random effect out of five
BellossomGrass WhistlePuts nearby opponents to sleep
GardevoirReflectA sphere that reflects projectiles
MetagrossEarthquakeShakes the ground, launching or burying fighters
AbomasnowIcy Wind, Ice PunchFreezes the careless
SnivyRazor LeafFires leaves in a straight line
OshawottSurfSweeps across the stage, carrying fighters along
ZoroarkFury SwipesSlashes in the air then slams down; unlocked through 10-Man Smash on 3DS
ChespinSeed BombExploding seeds
FennekinIncinerateColumns of flames
FletchlingPeckChases and pecks a player
SpewpaStun SporeA paralyzing cloud when it is hit
GogoatTake DownA mount to ride
SwirlixCotton SporeSlows down opponents
InkayTopsy-TurvyMakes opponents fall
DedenneDischargeAn X-shaped discharge

The Pokémon of Master Balls

PokémonMoveEffect
MoltresFlyRises up, hurting on contact
Mew—Flies off, dropping rare items
EnteiFire SpinA column of fire
SuicuneAurora BeamA horizontal beam
LugiaAeroblastA tornado coming from the background
Latias and LatiosSteel WingFly across the screen at full speed
KyogreHydro PumpPushes fighters toward the edge
DeoxysHyper BeamA laser aimed at the ground
PalkiaSpacial RendFlips the screen and reverses the controls
GiratinaDragon BreathHorizontal tornadoes
DarkraiDark VoidPulls in and puts to sleep
ArceusGravityMakes airborne fighters fall
VictiniVictory StarGives the player its ability
KyuremGlaciateFreezes nearby fighters
KeldeoSecret SwordTeleports and strikes with its horn
MeloettaEchoed VoiceNotes that hurt; unlocked after 8 hours of play on 3DS
GenesectTechno BlastA shot that passes through walls and objects
XerneasGeomancyPowers up everyone, especially whoever threw the ball