Nauru
The Nauru Reed-warbler Acrocephalus rehsei (BirdLife Int'l) is often considered distinct from the Nightingale Reed-warbler Acrocephalus luscinius (ARKive) and is found nowhere else.
A subspecies of skink Emoia
arnoensis nauru (Internet
Archive) is also endemic.
Invertebrates known solely from Nauru include an intertidal dwarf bug Corallocoris (or Omania) nauruensis (a
related species is illustrated on p. 254 of CDL
pdf file) and a barnacle Armatoglyptes
(or Lithoglyptes)
echinoideus
(WoRMS).
Although it has been claimed that a species of Phyllanthus might
be endemic, according to "The Flora of Nauru" (Smithsonian
9 MB pdf file), there are no endemic vascular plants and the claimed
endemic turned out to belong to the widespread Phyllanthus societatis
(Tiaredex).
Nauru is part of the Eastern Micronesia Tropical Moist Forests
ecoregion (EoE).