Question

How to make thunderus change its forme?

Comments

You obtain an item called the Reveal Glass, and use it on any of the 3 Djinn to transform them to their Therian forms.  Use it again to change them back.

Unfortunately, I don't remember what city it's in (hence why this is a comment, not an answer), but there is a glass workshop.  You talk to one of the people inside while any 1 of the 3 Djinn Pokemon are in your party, and they will give you the Reveal Glass.

Answer 1

Fly to the Shalor city in X or Y, and walk south into the cave and speak to the lady just inside. She'll activate a dialogue if thunderous or the other two are in your party. I don't remember if they just transform or if she gives the mirror as a result.
As for black and white, I remember the village being in the hills north of undella bay resort, but you need waterfall to get there, and you cannot fly there.

Answer 2

>The forms of Tornadus, Thundurus and Landorus are fully capable of being changed. This is done using the item, Reveal Glass, which is obtained by taking the Therian Forme Landorus to Abundant Shrine. Then, when you use the item, it will switch the Pokémon between forms. If you change the Pokémon obtained in Dream Radar to the Incarnate Form, they will have their Hidden Ability. However, you can also change Incarnate Form Tornadus/Thundurus/Landorus captured in Black & White into their Therian Forme. ~ Serebii

Breakdown
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First, you need to obtain Landorus-Therian from the Dream Radar. Then, take it to the Abundant Shrine, where you will receive the Reveal Glass. You can use the Reveal Glass to change Thundurus, Landorus and Tornadus to and from their Therian forms.

Comments

Is the Dream Radar even a thing anymore? I would have thought that it would have stopped when the Wi-Fi shut down for DS games.

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The Dream World was shut down, but wasn't the Dream Radar a separate thing?

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Dream Radar is separate. I know because I have Dream Radar but not the Dream World.

@Sep: It doesn't use Wi-Fi iirc, it just transfers directly to the game card.

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Yes, it was, but didn't the Dream Radar use Nintendo WFC? I've never used it so I don't know.

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See edited comment above ^^

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Oh, okay.
Does that mean that you can still buy and use it?