Trap Monsters is a semi-official term used to describe Trap Cards with effects that Special Summon themselves as a Monster Card. In some cases, once the monster has been Summoned, it is both a monster and a Trap Card simultaneously, and in other cases, the Trap Card will simply Special Summon itself as solely a monster card. The card's text will state which is the case.
Depending on the specifics of the effect that Special Summons it, the Trap Monster may Special Summon itself from the Spell & Trap Zone (when the card is activated) or the Graveyard (due to the Trap Card's effect). Whether or not the card is still treated as a Trap Card also depends on the effect that Special Summoned it.
Still a Trap Card
Trap Monsters that are still treated as Trap Cards after being Summoned include the text "(This card is also still a Trap.)" as part of the effect that Special Summons them. All such Trap Monsters are Continuous Traps, and are exclusively Special Summoned from the Spell & Trap Zone, at the activation of the Trap Card.
As they are considered both monsters and Trap Cards, these Trap Monsters are affected by effects that affect monsters (e.g. "Dark Hole") and effects that affect Traps (e.g. "Harpie's Feather Duster").
Despite being Trap Cards, after being Summoned they only use a Monster Zone and do not use a Spell & Trap Zone. (Prior to the Master Rules April 1st 2020 Revision, they used both a Monster Zone and a Spell & Trap Zone after being Summoned. However, despite preventing a Spell & Trap Zone from being used, the Trap Monster's card was placed in a Monster Zone and was not considered to be in the Spell & Trap Zone.)
Interactions with activation conditions
If a card or effect that specifically destroys monsters (e.g. "Offerings to the Doomed", "Mirror Force") is activated to destroy such a Trap Monster, cards that can only be activated when a monster would be destroyed (e.g. "My Body as a Shield") can be activated, but cards that can only activate when a Trap would be destroyed (e.g. "Fake Trap") cannot be activated. Likewise, if a card or effect that specifically destroys Traps (e.g. "Mystical Space Typhoon", "Heavy Storm") is activated to destroy such a Trap Monster, cards that only activate in response to effects that would destroy monsters can be activated, but effects that only activate in response to effects that destroy Trap Cards cannot. If a card or effect is activated that can destroy either monsters or Trap Cards (e.g. "Raigeki Break"), either of the aforementioned cards can be activated in response.
Returning to the Spell & Trap Zone
If such a Trap Monster's Trap effects are negated (e.g. due to "Jinzo", "Royal Decree"), that Trap Monster returns to the Spell & Trap Zone as a face-up Continuous Trap Card, and remains there face-up—only treated as a Trap Card—even if its effects later cease to be negated.
In either case, if there are no available Spell & Trap Zones for the Trap Monster to be placed in when it is returned there, then the Trap Monster is sent to the Graveyard via game mechanics.
Not a Trap Card
Trap Monsters that are not treated as Trap Cards after being Summoned include the text "(This card is NOT treated as a Trap.)" as part of the effect that Special Summons them. These Trap Monsters either have an effect that Special Summons them from the Graveyard as a monster (in which case they can be either a Normal Trap or Continuous Trap), or have an effect that Special Summons them upon their card activation (in which case they are always a Normal Trap). All Trap Monsters that Special Summon themselves from the Graveyard specifically are not treated as Traps after being Summoned.
As they are only considered monsters, these Trap Monsters are only affected by effects that affect monsters. If such a Trap Monster is flipped face-down after being Summoned, it is Set in the Monster Zone like any other monster.
After being Summoned, they only use a Monster Zone and do not use a Spell & Trap Zone. (This was the case even before the Master Rules April 1st 2020 Revision.)
Trap Monsters, whether as members of a distinct archetype (such as "Golden Land" Continuous Trap Cards) or simply generic cards (such as "Statue of Anguish Pattern"), form a slow control-based strategy that attempts to maximize card advantage or some form of utility from each Trap Monster Summoned: whether the individual Trap has its own effect on Summon, or another card, such as "Anguish Pattern", generates support advantage and/or removal.
List of Trap Monsters
|D/D Cold Golem||???||???||???||1||300||???|
|D/D Greedy Golem||???||???||???||???||?||???|
|Death Spirit Zoma||???||???||???||???||1800||500|
|Embodiment of Apophis||Normal Monster||Reptile||EARTH||4||1600||1800|
|Metal Reflect Slime||Normal Monster||???||???||???||0||?|
|Phantom Knights Dark Gauntlet||Effect Monster||???||???||4||?||600|
|Phantom Knights Shade Brigandine||???||???||???||4||?||800|
|Stronghold the Moving Fortress||Effect Monster||Machine||EARTH||4||0||2000|
- "ストレートフラッシュ | カードに関連するＱ＆Ａ" [Straight Flush | Related Q&A]. 遊戯王OCGカードデータベース (in Japanese). Konami. 5 February 2015. Retrieved 7 May 2022.
自身の効果によってモンスターゾーンに存在する「アポピスの化神」は魔法＆罠ゾーンに存在するカードではありません。（「ストレートフラッシュ」を発動する事自体ができません。）["Embodiment of Apophis" in the Monster Zone due to its own effect is not a card in the Spell & Trap Zone.]