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
Pokémon Move Effect
Meowth Pay Day Pelts the nearest opponent with coins
Electrode Explosion Explodes after a delay; it can be thrown again before then
Goldeen Splash Splashes… to no effect
Staryu Swift Fires stars at the enemy
Eevee Tackle Leaps from left to right
Snorlax Body Slam Grows and lands on a wide area
Togepi Metronome A random effect out of five
Bellossom Grass Whistle Puts nearby opponents to sleep
Gardevoir Reflect A sphere that reflects projectiles
Metagross Earthquake Shakes the ground, launching or burying fighters
Abomasnow Icy Wind, Ice Punch Freezes the careless
Snivy Razor Leaf Fires leaves in a straight line
Oshawott Surf Sweeps across the stage, carrying fighters along
Zoroark Fury Swipes Slashes in the air then slams down; unlocked through 10-Man Smash on 3DS
Chespin Seed Bomb Exploding seeds
Fennekin Incinerate Columns of flames
Fletchling Peck Chases and pecks a player
Spewpa Stun Spore A paralyzing cloud when it is hit
Gogoat Take Down A mount to ride
Swirlix Cotton Spore Slows down opponents
Inkay Topsy-Turvy Makes opponents fall
Dedenne Discharge An X-shaped discharge
Meowth. Electrode.
The Pokémon of Master Balls
Pokémon Move Effect
Moltres Fly Rises up, hurting on contact
Mew — Flies off, dropping rare items
Entei Fire Spin A column of fire
Suicune Aurora Beam A horizontal beam
Lugia Aeroblast A tornado coming from the background
Latias and Latios Steel Wing Fly across the screen at full speed
Kyogre Hydro Pump Pushes fighters toward the edge
Deoxys Hyper Beam A laser aimed at the ground
Palkia Spacial Rend Flips the screen and reverses the controls
Giratina Dragon Breath Horizontal tornadoes
Darkrai Dark Void Pulls in and puts to sleep
Arceus Gravity Makes airborne fighters fall
Victini Victory Star Gives the player its ability
Kyurem Glaciate Freezes nearby fighters
Keldeo Secret Sword Teleports and strikes with its horn
Meloetta Echoed Voice Notes that hurt; unlocked after 8 hours of play on 3DS
Genesect Techno Blast A shot that passes through walls and objects
Xerneas Geomancy Powers up everyone, especially whoever threw the ball