Thunder Dragon

 This card has a retrained counterpart: Thunder Dragondark.

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Thunder Dragon Titan is a three-headed version of this monster.

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This card appears in the Japanese artwork of Remote Revenge.

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In Yu-Gi-Oh! GX Spirit Caller, Thunder Dragon can be discarded, even if the user has no more copies in their Deck.

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This card has the same ATK, Attribute and Type as Thunder Sea Horse and a similar name and effect.

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One way of saying seahorse in Japanese is tatsu no otoshigo (竜の落し子) which literally means dragon's illegitimate child, which may allude to this similarity.

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OCG Rulings

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The effect of "Chain Energy" is applied even when a Spell/Trap Card is Set from the hand, or when the effect of "Kuriboh"/"Thunder Dragon"/etc is activated.: Spell Card > Chain Energy

Discarding this card is a cost.: Effect Monster > Thunder Dragon: Does the effect of "Thunder Dragon" activate when "Thunder Dragon" is discarded by a Spell/Trap Card?

You can add either 1 or 2 cards to your hand.: When the effect of "Thunder Dragon" is activated, must you add a card from your deck to your hand? You cannot choose to add 0 cards to your hand.

If there are no copies of "Thunder Dragon" in your deck, then you shuffle your deck and end resolution.

If "Thunder Dragon" is discarding by a Spell/Trap Card, its effect does not activate because it was not discarded to activate its effect.

"Light and Darkness Dragon" will also negate the effect of a monster which activates in the hand. The monster will not be sent to the Graveyard.: Can the effect of "Light and Darkness Dragon" negate the effect of a monster like "Gorz the Emissary of Darkness" which activates in the hand?
If "Kuriboh" or "Thunder Dragon" has its effect negated by "Light and Darkness Dragon", then it was discarded as a cost, so it does not return to the hand.

"Angel O7" prevents effects from activating or being activated from any Zone. The effect of "Sangan" will not activate. The effect of "Thunder Dragon"/"Kuriboh"/etc cannot be activated.: Can you activate an effect from the Hand or Graveyard while the effect of "Angel O7" is being applied?

You cannot activate the effect of "Thunder Dragon" if there are no copies of "Thunder Dragon" in your Deck.: Can the effect of "Thunder Dragon" be activated if there are no copies of "Thunder Dragon" in your Deck?

Mentions in Other Rulings

When the Turn Player Normal Summons, Flip Summons, or Special Summons a monster, the Turn Player cannot first activate the Ignition Effect of a monster in his hand (like "Thunder Dragon" or "Genex Ally Birdman") nor the Ignition Effect of a monster in his Graveyard (like "Plaguespreader Zombie" or "Blackwing - Vayu the Emblem of Honor

Previously Official Rulings

You can discard "Thunder Dragon" to activate its effect even if it would not go to your Graveyard. For example, when "Banisher of the Light" is face-up on the field.

If you have your opponent's "Thunder Dragon" in your hand because of "Exchange", you can discard it to activate its effect and search your Deck for up to 2 "Thunder Dragons".

You cannot discard "Thunder Dragon" to activate its effect if you do not have any "Thunder Dragon" in your Deck.

Mentions in Other Rulings

Banisher of the Light: "Kuriboh" and "Thunder Dragon" can be discarded from your hand and activated while "Banisher of the Light" is on the field.

Gravekeeper's Watcher: You cannot activate "Gravekeeper's Watcher" when a card is discarded as a cost, such as "Magic Jammer", "Kuriboh", "Tribute to The Doomed", "Thunder Dragon", or "Type Zero Magic Crusher".

Prohibition: If "Thunder Dragon" is declared with "Prohibition", it can be discarded with "Card Destruction" but it cannot be discarded to activate its effect.