How many of the Ainur were unable to change their physical form?

Question

The Ainur are spirits who do not need bodies, but they can assume them. From the Ainulindalë in the Silmarillion:

Their shape comes of their knowledge of the visible World, rather than of the World itself; and they need it not, save only as we use raiment, and yet we may be naked and suffer no loss of our being. ... at times they may clothe themselves in their own thought, made visible in forms of majesty and dread.

An Ainu can lose the ability to assume physical form if it loses enough of its power—if I’m not mistaken, this happened to Sauron.

The Wizards were apparently bound to particular physical bodies while they were on assignment in Middle-earth. From this answer on another question, quoting Letter 156:

By ‘incarnate’ I mean they were embodied in physical bodies capable of pain, and weariness, and of afflicting the spirit with physical fear, and of being ‘killed’, though supported by the angelic spirit they might endure long, and only show slowly the wearing of care and labour.

But what about other Ainur? For example, here is a vague physical description of the Balrogs from chapter three of the Silmarillion:

Their hearts were of fire, but they were cloaked in darkness, and terror went before them; they had whips of flame.

The implication is that they always looked like that. And from the answer to this question, various other creatures are known or suspected to be Maiar, and not all of those creatures were observed to change or shed their physical form.

Were some Maiar other than the Wizards bound to/stuck in particular physical forms? Is there any reason a Balrog couldn’t discard his body, or temporarily assume the form of a wolf? As the Ages wore on, perhaps fewer and fewer Maiar could change form at will, having spent their power over the millennia, but it seems like many of the Maiar never changed form even in the early Ages.

Answer 1

In principle, we can assume that any Maiar that can be "killed" is stuck in their particular form; otherwise, what would be the point in destroying their bodies?

This makes things somewhat easier, because those Maiar who have been killed is a relatively short list.

The Balrogs

The big ones, of course. A possible reason for their being stuck in Balrog-shape is put forth in the Ósanwe-kenta, a manuscript related to the "Quendi and Eldar" essay (published in The War of the Jewels), which was first published in the 1998 issue of Vinyar Tengwar:

"Here Pengolodh adds a long note on the use of hroar by the Valar. In brief he says that though in origin a 'self-arraying', it may tend to approach the state of 'incarnation', especially with the lesser members of that order (the Maiar). 'It is said that the longer and the more the same hroa is used, the greater is the bond of habit, and the less do the 'self-arrayed' desire to leave it.'

[...]

[Melkor's greatest servants] became wedded to the forms of their evil deeds, and if these bodies were taken from them or destroyed, they were nullified, until they had rebuilt a semblance of their former habitations, with which they could continue the evil courses in which they had become fixed.

Basically, the spiritual beings become "addicted" to the forms they use, and over time it becomes harder and harder to give it up. The Maiar are more susceptible to this phenomenon than the Valar are, which somewhat explains why this happened so much more quickly to the balrogs than it did to Morgoth - he had to disseminate the majority of his power into Arda, while they just needed to forget to change out of their balrog suits after a long aeon of balrogging it up.

I will admit that this isn't a completely satisfying explanation; we wouldn't expect the balrogs to have used their physical form all the time, after all, which makes it unclear how exactly this process worked. It is, however, all we really have to go on.

Huan

This one we don't know. Shamshiel points out elsewhere that Huan is most likely a Maiar, but we know that:

• He is killed
• He doesn't change shape in situations where it would be useful for him to do so

Which leaves us a little stuck. One possibility is that he was something like the Wizards, bound to a physical form in order to fulfil a specific purpose in Middle-earth. However, this would seem to be contradicted by the note Shamshiel cites, which says that Huan "robed himself":

Living things in Aman. As the Valar would robe themselves like the Children, many of the Maiar robed themselves like other lesser living things, as trees, flowers, beasts. (Huan.)

^{History of Middle-earth X Morgoth's Ring Part V: "Myths Transformed"}

Which implies one of two possibilities:

1. Like the balrogs, he was simply stuck in a form he'd been using for thousands of years
2. He kept his wolfhound shape voluntarily throughout his time in Middle-earth

There's no evidence for or against either, but personally I think the first is vastly more likely.

Melian?

I put a question mark because we don't know if Melian was bound to her physical form. If we assume that Huan was, it seems likely; she was in that form for longer than he was in his, after all (so far as we know, anyway).

There is some support for this from the text, but it's a little shaky:

Melian was of the divine race of the Valar, and she was a Maia of great power and wisdom; but for love of Elwë Singollo she took upon herself the form of the Elder Children of Ilúvatar, and in that union she became bound by the chain and trammels of the flesh of Arda.

^{The Silmarillion III Quenta Silmarillion Chapter 22: "Of the Ruin of Doriath"}

One possible reading of this passage is that Melian has literally become bound up in the physical matter of Arda - which seems to imply that she's become stuck in her body.

Further supporting this interpretation is Ósanwe-kenta, a manuscript related to the "Quendi and Eldar" essay published in The War of the Jewels. In this manuscript, Tolkien writes (bold is my emphasis, italic is his):

Pengolodh also cites the opinion that if a "spirit" (that is, one of those not embodied by creation) uses a hröa for the furtherance of its personal purposes, or (still more) for the enjoyment of bodily faculties, it finds it increasingly difficult to operate without the hröa. The things that are most binding are those that in the Incarnate have to do with the life of the hröa itself, its sustenance and its propagation. Thus eating and drinking are binding, but not the delight in beauty of sound or form. Most binding is begetting or conceiving.

"We do not know the axani (laws, rules, as primarily proceeding from Eru) that were laid down upon the Valar with particular reference to their state, but it seems clear that there was no axan against these things. Nonetheless it appears to be an axan, or maybe necessary consequence, that if they are done, then the spirit must dwell in the body that it used, and be under the same necessities as the Incarnate. The only case that is known in the histories of the Eldar is that of Melian who became the spouse of King Elu-thingol.

However, we learn later that she returns to Aman; how she could have done this while being stuck in one form is unknown.

Others

As I said before, any Maiar-like creature that can be killed can, presumably, be said to be stuck in a physical form; so we can extend that list to include an unknown subset of the following list:

• Orc-form Maiar
• Possibly the Ents
• Possible the Eagles
• Possibly the Werewolves
• Possibly the Giant Spiders
• Possibly Dragons

If we accept that "staying in one form for a long time" makes you become stuck to that form, then we may have to include Tilion, the Moon; although we don't know if he had to incarnate in order to fill his role¹. If he did, then presumably he's been stuck in that form since the start of the First Age.

We may also have to include other named Maiar (such as Eönwë, or Ossë); the fact that they were known to the Eldar indicates that they had some consistent form that they used, and Tolkien's quote above suggests that frequent use of the same form also causes one to become "stuck." However, we cannot confirm that this happened to any of the other Maiar; we're never told so, and we just don't have enough information on how the process works.

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¹ Though we know Arien, the Sun, did shed her physical form in order to become the Sun. Thanks to Mithoron for correcting me on this