Some of these require the use of a special taunt before activating. I haven't tested all characters, but I know Marth's and Captain Falcon's are done by pressing Up Taunt, Down Taunt, and then Up Taunt again in rapid succession. Will confirm later.
Please note, these specials don't seem to have official names.
Format
Character (Condition) - How to activate
Mario (Self 160% or over) - After a successful back throw, side-taunt to activate Metal Mario
Peach (Self 160% or over) - After a successful back throw, forward throw to activate Ultimate Peach Slap
Yoshi (Self 160% or over) - After a successful forward throw, do Up Taunt to activate Dragon Breath
Diddy (Self 160% or over) - After a successful back throw, do Neutral-B while on the ground to activate Bullet Time
Falcon (Special Taunt) - 1. Up-Smash is modified and 2. Aerial Down-B becomes Inazuma Kick
Fox & Falco (Self 160%~ 199%) - After a successful back throw, do a side taunt to activate Time Stop. Inflicts 40% damage on self on activation.
Wolf (Self 150% or over) - After a successful forward throw, do an Up Throw while holding B to activate Lethal Rampage
Link (Special Taunt) - Side Smash's second hit will activate Hyrulian Flash
Toon Link (Self 150% or over) - After a successful back throw, perform an Aerial Up B while Holding A to activate Emerald Twister (Note: It is
possible to control the twister)
Ganon (Self 160% or over) - After a successful back throw, perform an Up Throw to unleash Ganon Corridor
Ganon (Special Taunt) - use Side B while on Ground
Kirby (No Condition) - Doing an Up Taunt will get you a Dragoon Part
Metaknight (Special Taunt) - A successful hit with Down B will activate Galaxia Darkness
King Dedede (Self 160% or over) - After a successful back throw, do a Down Smash to let loose Burning Hammer Throw
King Dedede (Self 200% or over) - After a successful forward throw, do an Up Taunt to summon either Lugia, Manaphy, or Deoxys
Pit (No Condition) - Doing a Down Taunt will double your next arrow's damage; this effect can be stored up to 4 times in a row
Ness (Self 160% or over) - After a successful back throw, doing a Down Taunt will heal back 50% of Ness's health. It can also heal beyond 0%, effectively creating a 50% barrier if used right after respawning.