Bell’s Woodnymph (Thalurania belli)
Bell’s Woodnymph is a hypothetical, yet very likely real species of hummingbird that apparently was endemic to the island of Dominica in the Lesser Antilles, where it seems to have been restricted to the rainforests of the highest mountains.
The species might have been on the brink of extinction when it was discovered and described by Alpheus Hyatt Verrill, an American zoologist, in 1905 (?).:
“Upperparts rich iridescent metallic green, becoming deep peacock blue or verditer-green on ferhead and crown; coppery on shoulders and deep bluish or emerald green on rump: scapulars, upperwing-coverts and uppertail-coverts, deep peacock or bluish green. Wings metallic purple or steel blue, the outer web of outer primary narrowly edged with white or pale ash grey. Basal portion of tail dull copper green, the outer half deep steel blue with violet reflections. The three outer feathers on each side broadly tipped with white and the outermost feather white at base also. Lower parts uniform snow white or less washed with greyish on flanks and sides. Flanks and sides beneath wings spotted with isolated bright green feathers. Ear-coverts and loral region deep velvety black in marked contrast to green of occiput. Bill dusky black with lower mandible slightly lighter near base.” 
No one else did ever see this species and it apparently was extinct shortly after the abovementioned account.
The description appears to be very extensive and correct, and I personally have no reason to doubt the former existence of such a species.
 A. Hyatt Verrill: Additions to the avifauna of Dominica. Notes on species hitherto unrecorded with descriptions of three new species and a list of all birds now known to occur on the island. 1905?
 Julian P. Hume: Extinct Birds: 2nd edition 2017