Linné (fils) designated the Doubtful Amaryllis as Amaryllis Equestris in his unpublished MS, and was followed by Aiton who called it A. equestre in the first edition of Hortus Kewensis.
Baker (1888) regarded H. barbatum as a color variant of H. equestris, though subsequent research indicates that it is indeed distinct from the red flowered form.
Redoute's portrait of Amaryllis equestre agrees with Amaryllis Belladonna (Linn.), and Amaryllis Reginae (Lin. fils).