I ran across this word in my New Great Russian-English Dictionary, where I happened to notice (near the bottom of p. 2912) this entry:
уна́у m indecl zool unau, two-towed sloth (Choloepus didactylus).
First, of course, I was amused by the “two-towed”; then I wondered about the word. The OED entry is from 1921:
Pronunciation: Brit. /ˈjuːnɔː/ , U.S. /ˈjʊˌnaʊ/
Etymology: Brazilian of the Island of Maranhão.
The South American two-toed sloth, Cholopus didactylus.
Adopted by Buffon from C. d’Abbeville Mission des Pères Capucins, etc. (1614) 252. Of the two kinds there mentioned by the names of Unaü and Unaü ouassou the former is Buffon’s Ai, the latter his Unau.
1774 O. Goldsmith Hist. Earth IV. 343 Of the sloth there are two different kinds,..the one, which in its native country is called the unan [sic], having only two claws upon each foot.
1834 H. McMurtrie tr. G. de Cuvier Animal Kingdom (abridged ed.) 93 Only one species [of Bradypus] is known, the Unau.., less uniform in its organisation than the Aï.
1872 G. M. Humphry Observ. Myology 21 A recess and dimple in the astragalus of Unau and of Aï.
The etymology is blatantly unsatisfactory; fortunately the AHD includes the word: “[Portuguese, from Tupí uná, lazy.]” And it also gives the pronunciation “oo’nou” (OO-now), which I shall adopt as my own, since I prefer to keep initial u- untainted by an unnecessary y- if I can.
The word doesn’t seem to have been used much in recent decades, but Google Books did turn up The Furry Animal Alphabet Book by Jerry Pallotta (1991),which devotes a whole page to it: “U is for Unau. The Unau is a two-toed sloth. Sloths are one of the slowest animals in the world….”