Estrangeiros benevolentes que (inicialmente) se recusam a se mostrar

Isso é quase certamente Childhood's End por Arthur C Clarke. Eu não vou revelar o final, já que isso seria um spoiler, mas é deixado como uma questão aberta se os alienígenas (conhecidos como os Senhores Supremos) eram realmente benevolentes.

Para citar a descrição da Wikipedia da primeira seção do romance:

In the late 20th century, the United States and the Soviet Union are competing to launch the first spaceship into orbit, for military purposes. When vast alien spaceships suddenly position themselves above Earth's principal cities, the space race ceases. After one week, the aliens announce they are assuming supervision of international affairs, to prevent humanity's extinction. They become known as the Overlords. In general, they let humans go on conducting their affairs in their own way. They overtly interfere only twice: in South Africa, where sometime before their arrival Apartheid had collapsed and was replaced with savage persecution of the white minority; and in Spain, where they put an end to bull fighting. Some humans are suspicious of the Overlords' benign intent, as they never visibly appear. The Overlord Karellen, the "Supervisor for Earth," who speaks directly only to Rikki Stormgren, the Secretary-General of the United Nations, tells Stormgren that the Overlords will reveal themselves in 50 years, when humanity will have become used to their presence. Stormgren smuggles a device onto Karellen's ship in an attempt to see Karellen's true form. He succeeds, is shocked, and chooses to keep silent.

Sua forma é aquela que inspira grande consternação.

It was a tribute to the Overlords' psychology, and to their careful years of preparation, that only a few people fainted. Yet there could have been fewer still, anywhere in the world, who did not feel the ancient terror brush for one awful instant against their minds before reason banished it forever.

There was no mistake. The leathery wings, the little horns, the barbed tail-all were there. The most terrible of all legends had come to life, out of the unknown past. Yet now it stood smiling, in ebon majesty, with the sunlight gleaming upon its tremendous body, and with a human child resting trustfully on either arm.

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Arthur C. Clarke escreveu uma história anterior que inspirou o Childhood's End, chamado de " Guardian Angel ". Nele encontramos uma raça de alienígenas que pretendem supervisionar a Terra e nos conduzir a uma grande civilização.

Apesar de nunca darmos uma boa olhada em um " Overlord ", o herói consegue dar uma olhada nas costas de um deles através de uma tela defumada.

Was it a lie? What had he really seen? No more, he was certain, than Karellen had intended. Me was as sure as he could be of anything that the Supervisor had known his plan from the beginning, and had foreseen every moment of it. Why else had that enormous chair been already empty when the circle of light blazed upon it? In the same moment he had started to swing the beam, but he was too late. The metal door, twice as high as a man, was closing swiftly when he first caught sight of it closing swiftly, yet not quite swiftly enough.
Karellen had trusted him, had not wished him to go down into the long evening of his life still haunted by a mystery he could never solve. Karellen dared not defy the unknown power above him (was he of that same race, too?) but he had done all that he could. If he had disobeyed Him, He could never prove it. 'We have had our failures.'
Yes, Karellen, that was true - and were you the one who failed, before the dawn of human history? Even in fifty years, could you overcome the power of all the myths and legends of the world?
Yet Stormgren knew there would be no second failure. When the two races met again, the Overlords would have won the trust and friendship of Mankind, and not even the shock of recognition could undo that work.

E, claro, a finalização da torção

And Stormgren knew also that the last thing he would ever see as he closed his eyes on life, would be that swiftly turning door, and the long black tail disappearing behind it.
A very famous and unexpectedly beautiful tail.
A barbed tail.