But, to get back to the Roc, it is, of course, mythical, but the question arises regarding the origin of belief in it. It may go back to the mythical Indian bird Garuda, but there may also be an origin in more material form. On the island of Madagascar there was once a huge bird, unable to fly, called the Aepyornis and, more colloquially, the elephant bird. While this creature may have become extinct centuries ago, its eggs have remained and the Malagasys and the Arabs, noting their size, inferred the existence of a giant bird from them. Indeed, the Malagasys have a giant bird, the vorompatra, in their legends and they may have passed traditions of that on to the Arabs.
Also a possibility is the memory of another extinct bird of Madagascar, the Malagasy crowned eagle (Stephanoaetus mahery), which was not the sort of creature you would want to run into if off meandering.
The Roc, because of its appearances in the Arabian Nights, has etched itself a place in human consciousness. It is a spectacular mythical bird, but, take my word, you wouldn't want one living near you.