Mangarevan Flycatcher (Pomarea gambieranus)
This very enigmatic bird was found in the year 1840 on the island of Mangareva, the largest of the Gambier Islands, and was described in the year 1844 as Lanius gambieranus.
The sole known specimen does not exist anymore, as does the only depiction of this specimen. The description is not very meaningful, the bird had a size of 14 cm, was brown on the upper side, yellowish colored on the underside.
In my opinion, it is quite possible that this is indeed a monarch flycatcher species, possibly from the genus Pomarea or alternatively from another, yet completely distinct, now extinct genus.
 J.-C. Thibault; A. Cibois: From early Polynesian settlements to present: bird extinctions in the Gambier Islands. Pacific Science 66(3): 1-26. 2011