Countries with Five Letters Quiz

And it apparently seems most likely to not be based on Latin. From The Wikpedia article (and has both pronunciations):

The name comes from the Niger River which flows through the west of the country. The origin of the river's name is uncertain. Alexandrian geographer Ptolemy wrote descriptions of the wadi Gir (in neighbouring modern Algeria) and the Ni-Gir 'Lower Gir' to the south, possibly referring to the Niger River. The modern spelling Niger was first recorded by Berber scholar Leo Africanus in 1550, possibly derived from the Tuareg phrase the (e)gărăw-n-gărăwăn meaning 'river of rivers'. There is broad consensus among linguists that it does not derive from the Latin niger 'black' as was first erroneously believed. The standard pronunciation in English is /niːˈʒɛər/, while in some Anglophone media /ˈnaɪdʒər/ is also used.