His identity is not given. Timaeus calls him the “poet and father” of all that comes to be. (28c)
It is one of the many likely stories (ton eikota mython) he tells:
So then, Socrates, if, in saying many things on many topics concerning gods and the birth of the all, we prove to be incapable of rendering speeches that are always and in all respects in agreement with themselves and drawn with precision, don’t be surprised. But if we provide likelihoods inferior to none, we should be well-pleased with them, remembering that I who speak as well as you my judges have a human nature, so that it’s fitting for us to be receptive to the likely story about these things and not search further for anything beyond it. (29c-d).
We are human beings, capable of telling likely stories, but incapable of discerning the truth of such things. In line with the dialogues theme of what is best, Timaeus proposes it is best to accept likely stories and not search for what is beyond the limits of our understanding.
It is essential to Socratic philosophy to be aware of our human limits. Traditionally, the poets tell the stories. The question raised in the Republic, of which the discussion in the Timaeus is a continuation of, is what are the best stories for the city that is to be the best city?