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Etymologically, "das Mädchen" is the diminutive of "die Magd/Maid" (cognate of the English word "maid" / "maiden"), which is no longer/rarely used in modern German. All diminutives ending in -chen are neuter: der Hund -> das Hündchen, die Katze -> das Kätzchen, das Haus -> das Häuschen
This is a predicate nominative. "es" and "ein Mädchen" refer to the same person. There's no object.