Qual é a criatura parecida com uma ave nas Duas Torres J.R.R. tampa?

Este design de capa é tirado de uma das capas sugeridas por Tolkien para o livro, proposta (mas rejeitada) para a impressão original:

Eu não sei se o próprio Tolkien comentou sobre isso, mas as fontes críticas mais respeitáveis concordam que é um Nazgûl:

• Humphrey Carpenter, em The Letters of J.R.R. Tolkien (ênfase minha):

  Orthanc is shown as a black tower, three-homed (as seen in Pictures no. 27), and with the sign of the White Hand beside it; Minas Morgul is a white tower, with a thin waning moon above it, in reference to its original name. Minas Ithil, the Tower of the Rising Moon (The Fellowship of the Ring p. 257). Between the two towers a Nazgûl flies.

  ^{The Letters of J.R.R. Tolkien Notes 143}

• Wayne Hammond e Christina Scull em J.R.R. Tolkien: Artista & Illustrator :

  Tolkien's final rendering of his The Two Towers dust jacket¹ is a moody painting in black, red, white, and grey on grey-brown paper, with sharp contrasts of dark and light. He simplified the composition of [his previous version], omitting one of the Nazgûl, the staff and the key upon Orthanc, and the single ring, but added further symbolic details.

  ^{J.R.R. Tolkien: Artist & Illustrator Chapter 5: "The Lord of the Rings"}

¹ Refere-se à primeira imagem postada nesta resposta

Era um dos Nazgûl, como declarado no capítulo 11 do livro 3, The Palantir . Embora a própria criatura (como mencionado por @vsz nos comentários) seja uma das feras caídas, ou falcões do inferno ¹ , montada pelos Nazgûl após os eventos nos Vaus de Bruinen.

At that moment a shadow fell over them. The bright moonlight seemed to be suddenly cut off. Several of the Riders cried out, and crouched, holding their arms above their heads, as if to ward off a blow from above: a blind fear and a deadly cold fell on them. Cowering they looked up. Avast [sic] winged shape passed over the moon like a black cloud. It wheeled and went north, flying at a speed greater than any wind of Middle-earth.

[...]

Nazgûl!’ he cried. ‘The messenger of Mordor. The storm is coming. The Nazgûl have crossed the River! Ride, ride! Wait not for the dawn! Let not the swift wait for the slow! Ride!
_{The Lord of the Rings: The Two Towers - Chapter 11, The Palantir}

O Nazgûl estava voando em direção a Orthanc vindo de Minas Morgul.

No Mapa interativo do Projeto SdA , podemos ver que no dia 5 de março, quando eles estavam acampando em Dol Baran , e os Nazgûl" rodaram para o norte "em direcção a Orthanc

Algumas citações para apoiar o acima são encontradas nas Duas Torres e no Conto dos Anos.

Winged Nazgûl passes over the camp at Dol Baran.
_{The Lord of the Rings: The Return of the King - Appendix B, The Two Towers}

Less than an hour had passed, and they had reached the Fords of Isen and crossed them. The Mound of the Riders and its cold spears lay grey behind them.

^{Créditos para @MartinSmith para o nome da criatura.}

As outras respostas afirmam que este é um Nazgûl (AKA Ringwraith, Black Rider, Dark Rider)

@vsz aponta

Isn't the Nazgûl the name of the guy riding on the flying beast, instead of the flying beast itself?

Parece que não há um nome canônico para as criaturas que os Nazgûl voaram.

Estados da Wikipédia

After losing their horses at the Ford of Bruinen, the Nazgûl returned to Mordor and reappeared mounted on hideous flying beasts; Beregond called them "Hell Hawks". Tolkien describes them as "fell beasts", though Tolkien applies the adjective fell ("fierce, cruel") to a variety of other creatures throughout The Lord of the Rings — even at one point to Gandalf. In a letter, he calls the winged mounts "Nazgûl-birds". In the absence of a proper name, derivative works sometimes press "fellbeast" or "fell-beast" into service.

O wiki da LOTR também discute isso em seu artigo sobre Fellbeast

There is a common misconception that these creatures are called Nazgûl, probably due to the fact that the unit in many video games (combined Fell beast and Ringwraith) was called a Nazgûl...