This is indeed a large scale mirror with elaborate swan's neck pediment flanking an elaborate plinth wherein the Phoenix bird spreads its wings. This mirror exhibits a plinth that is rarely seen in these mirrors. In the four examples shown below, none of them have such an elaborate plinth. This is a grand mirror, and all original.
The mirror is a fine specimen, and retains a later beveled glass of good age. Mirrors such as this, also called "Constitution mirrors" are highly desirable and select examples appear in the Diplomatic Receptions Rooms of the United States Department of State. Two such examples at the Department of State are illustrated in The Mirror Book, by Herbert Shiffer as well as two other closely related examples (see scans below).