What a Knightmare!
This monster is currently seeing heavy side-deck play in order to counter Drytron.
Ally of Justice Cycle Reader
Infernoble
Infernoble Arms - Durendal / Hauteclere / Joyeuse
These monsters facilitated an era of abuse in FTK-s and many dark-focused decks with the ability to spam the board and generate card advantage.
Dangers! (Archetype)
Don't forget to use this Fusion monster's graveyard effect in the EP, Nuhdeer.
Titaniklad the Ash Dragon
Reprinted in The Order of the Spellcasters structure deck, this powerful hand trap acts as a floodgate for searching and has seen usage since it's printing in Starstrike Blast.
Droll & Lock Bird
Doppel Token
Doppelwarrior
This monster, who's alternate form sees play in Duel Links, has had it's original artwork changed both in the TCG and anime.
Barrel Dragon
Spyral
Spyral GEAR - Big Red / Fully Armed
This card, most commonly played in Infernoid, has an entire GOAT deck named after it, along with another similarly natured card.
Reasoning
This pseudo-Kaiju makes up for him outing your boss monster with a present!
Santa Claws
This ghost girl was the only one to have it's artwork censored due to some religious imagery.
Ghost Sister & Spooky Dogwood
Vague Shadow Tokens
Blackwing - Gofu the Vague Shadow
The printing of this card in Savage Strike allowed virtually any 2 monsters to end on the full orcust board until it's archetypal link monster, which was used to summon this monster, was banned.
Orcustrated Knightmare
D.D.
D.D.R - Different Dimension Reincarnation
Played in Speed Duels, this card has 2 positive effects depending on what you roll, but its third negative downside can be mitigated with the usage of a Joey skill, which is what it was primarily paired with.
Dicephoon
Because of old-school FTK shenanigans and self-looping, this fire beast-warrior was semi-limited for a period of time for being able to revive itself over and over.
Manticore of Darkness
A ritual spell for one of the most powerful and popular graveyard archetypes of all time, this card allowed you to not only summon their ghost rare ritual monster, but also with it's second lesser-known effect search monsters of its archetype from deck.
Doomsday Token
Fires of Doomsday
Responsible for infinite loops with Galis the Star Beast, this card remains limited to this day due to its FTK potential.
Genex Ally Birdman
Inzektor
Inzektor Axe - Zektahawk
Inzektor Crossbow - Zektarrow
Inzektor Swod - Zektkalibur
Played in a specific variant of rank-up phantom knight, you would summon this powerful floodgate monster after ranking up Mechquiped Angineer with Astral Force.
Number 56: Pair-a-Dice Smasher
Ruling shenanigans and poor card text caused this monster to be enough of a nightmare that it would be limited solely because of its ruling complications until its errata.
Twin-Headed Behemoth
Played as an out to secret village of the spell casters, this monster sees niche competitive play despite its summoning restriction and lack of effect.
Clara & Rushka, the Ventriloduo
Moon Token
Arcana Force XVIII - The Moon
This monster saw early competitive play in Extra Deck Monarchs thanks to its level and ability to search, but was quickly revealed to be overhyped.
Sphere of Chaos
Vylon
Vylon Segment / Component / Material / Filament
The Ryan Lavine Theorem claims that this card can be played with a gentleman's agreement once Yu-Gi-Oh! Reaches a state of perfect optimization.
Cup of Ace
Played in an infamous stun deck last year that turbo-ed out Vanitys Ruler and Fiend, this card allowed for easy token generation for both you and your opponent.
Dragonoid Generator
This monster was given out as promotional material in order to help duelists snack on the go better.
Potato & Chips
Utchatzimime Token
Phantasmal Lord Ultimil Bisbaalkin