best thing to true polymorph dragon into

[Blood of Gaea]

This just opened up an interesting possibility. If you polymorph something into an object, the description specifically says that it retains no memory of being the object. It does not say that for new creature forms so, by inference, the target DOES remember its' time polymorphed into a dog.

If you plymorph the dragon into a dog, and then spend the next year or so taking good care of it, and training it up into a Good Dog, doesn't the dragon remember what it learned as a Very Good Boy? They retain their alignment and personality in the new form, so if the alignment and personality change while in the new form, wouldn't they retain that when they go back to their old form?

Would it not be unreasonable for a well-intentioned but slightly mad archmage to go around True Polymorphing dragons and demon lords into puppies, partnering them with needy children with whom they will form a strong, lifelong bond, and then, at the end of the dog's long, loving life with their human best friend, let them turn back to their monstrous true selves and see what lessons they've learned? How could this possibly go wrong?

The dog, deciding with its mind that is still the Dragon's (even if much less intelligent) decides it prefered to be a Dragon, and so jumps off a cliff, turning back into a dragon.

Now, I could see making the arguement that many enemies would not know that a permanent True Polymorph would allow you to turn back on death, but I would not expect an ancient dragon to be ignorant.

[Spriteless]

Keep it simple. Diamonds, used to resurrect one of the Dragon's victims. Then you ask the DM, is being consumed the same as being destroyed? Can we science this question?

[FabulousFizban]

a housecat, which will then be adopted by the party. An eeevil housecat.

[RossN]

Pseudodragon.

Familiar body type, familiar abilities, but an absolute mockery of its former strength and power. Sure, it retains its intelligence, and can even tell people what's going on, but the most important part is, no one will ever believe it. Of course a Pseudodragon would claim to be a powerful red dragon that was mercilessly stripped of its body and turned into this frail thing! Why wouldn't they claim that?

And for a Red Dragon especially, with all their pride and ego, nothing could be worse than being brought within sight of its own majesty and yet completely helpless to recover its former state.

I was just thinking that myself. Themathically it feels great and Pseudodragons are cute.

[Fable Wright]

An origami dragon.

Possibly a Tiny Servant in the form of an origami dragon, if you must have it remember its time in humiliation.

[kingcheesepants]

a housecat, which will then be adopted by the party. An eeevil housecat.

A similar idea to the dog, but without any chance of him ever turning good, after all is there such a thing as a nonevil housecat?

Keep it simple. Diamonds, used to resurrect one of the Dragon's victims. Then you ask the DM, is being consumed the same as being destroyed? Can we science this question?

Interesting to think about, I'm not sure how I would rule that myself. Probably the dragon disappears and is put into the aether.

The dog, deciding with its mind that is still the Dragon's (even if much less intelligent) decides it prefered to be a Dragon, and so jumps off a cliff, turning back into a dragon.

Now, I could see making the arguement that many enemies would not know that a permanent True Polymorph would allow you to turn back on death, but I would not expect an ancient dragon to be ignorant.

The dragon would almost certainly know about polymorph though whether he could as a dog put 2 and 2 together and would overcome the strong resistance to self destruction is another question entirely. We could cast mass suggestion on him just in case but I kinda feel it would most likely be unnecessary.
A dog or a psuedodragon feels like the best option. I like the idea of rehabilitating him as a dog, just humiliating him as a psuedodragon is appealing though. Though the psuedodragon would retain a 10 int which would certainly require management.