Shapes and Colors
Some say that Joshua Schmidt's hate for it led to the now-lapsed ban of this geometric monster, a one-card starter/extender whose name has become synonymous with legacy support that does everything.
Mathmech Circular
Galaxy-Eyes Photon Lord
Number 90
This LIGHT Machine component monster was once limited, and some say unfairly so. Despite its non-HOPT effect, many duelists believe it was limited to cynically avoid banning Firewall Dragon.
A-Assault Core
Against the Paradox Brothers, Yugi and Joey faced down this three-bodied boss monster that was once one of Yugioh's hardest and least rewarding to summon. At least the retrain is a contact fusions.
Gate Guardian
This once-banned backrow received an errata that prevented it from forcing draws or dealing game-winning amounts of damage. Probably for the best that Labrynth can't search a way to burn you 3000 for game.
Ring of Destruction
Herald of Orange Light crops up from time to time in Fairy decks as a catch-all monster negate handtrap. Less played are its two sibling cards--Purple Light negates Traps, and this one stops Spells.
Herald of Green Light
Dugares the Timeless
Number 60
Each K9 monster has an alphanumeric codename; this partner monster of Jokul shares its number, but has a different letter.
K9-66b Lantern
While its archetype now contains various fusion monsters and associated support, this C-list boss from Yugi's deck can only be summoned from the hand by tributing its three component monsters.
Valkyrion the Magna Warrior
Probably the card with the most amount of errata in the game, this Field Spell's effects can differ wildly depending on which printing of the card you're reading. The gist of it is always the same, though--no touching your Graveyard.
Necrovalley
This monster's archetype interacted with the physical placement of cards and still influences the way some duelists place their backrow today. This monster was perhaps the most important of its lot to play around, searching extra cards for every card your opponent had in its same column.
Mekk-Knight Blue Sky
Giant Hand
Number 106
E - Emergency Call is the most well-known of them, but there's a whole suite of spells that spell out HERO. H - Heated Heart, E - Emergency Call, R - Righteous Justice, and this reborn spell.
O - Oversoul
Despite actually being Fusion Monsters, the monsters of this quick-tagging sub-archetype don't actually list any Fusion Material, and can't even be Fusion Summoned. Those don't seem like Fusion Monsters to me!
Masked HERO
This DARK Fairy received a database errata before it even hit the shelves in TCG regions; due to a misprint, it would've searched Spell/Traps belonging to a different -- but closely related -- archetype than intended.
Aluber the Jester of Despia
This Pendulum Magician has a very peculiar interaction with negation--its non-HOPT on-destruction effect will trigger if its destroyed on the field and sent to the face-up Extra Deck, but if it's negated there it'll be destroyed and sent to the Graveyard, and then trigger again.
Purple Poison Magician
Big Eye
Number 11
Worms are widely considered one of Yugioh's worst archetypes ever, despite having this Trap card that triggers all their flip effects, draws you cards, and teleports out a monster on a delay. Too bad its unsearchable!
W Nebula Meteorite
Appearing in a Battle City tag duel, this multi-headed Fiend's TCG counterpart can by only be summoned by tributing 2 monsters, including either Grand Tiki Elder or Melchid the Four-Face Beast. That's basically just Fusion Material!
Masked Beast Des Gardius
This previously-banned Synchro monster, a standout of Edison format, was brought back into the game alongside an errata that rendered it almost completely unplayable despite adding only a single word to its text: "EARTH."
Goyo Guardian
This retro removal spell discards a card to target any monster and put it on top of the owner's deck.
Back to Square One
Utopic Dark Infinity
Number XX
Due to the way non-alphanumeric characters are checked, "A Case For K9" is the first card in the English database when sorted alphabetically. Before its release, though, that position belonged to this piece of Alien support.
"A" Cell Breeding Device
This evolution of Stardust Dragon actually belongs to another archetype entirely, representing the bond between Yusei and Bruno/Antinomy. Because of this, its the only level 10 or higher Synchro of its archetype that isn't a Machine.
Shooting Star Dragon T.G. EX
If I activated Primite Lordly Lord declaring the name Sky Scout, you'd probably be at least a little confused if I proceeded to summon an outdated copy of the card with this pre-errata name.
Harpie's Brother