Is the belief that the existence of any ancient being other than God with His attributes is inherently impossible, whether it is said to be ancient by itself or by another, a correct belief that aligns with the established understanding?

The ancient is either ancient by itself or ancient by another. That which is ancient by itself is necessarily existent by itself. As for that which is ancient by another, its existence is deemed impossible by most rational thinkers. Those who permit it say: it is ancient due to the ancientness of its necessitating cause, which is necessarily existent by itself, because it is an effect of a complete, necessitating, eternal cause that necessitates its effect.