From The Collaborative International Dictionary of English v.0.48:
Gnu \Gnu\, n. [Hottentot gnu, or nju: cf. F. gnou.] (Zool.)
One of two species of large South African antelopes of the
genus Catoblephas, having a mane and bushy tail, and curved
horns in both sexes. [Written also gnoo.]
[1913 Webster]
Note: The common gnu or wildebeest (Catoblephas gnu) is
plain brown; the brindled gnu or blue wildebeest (C.
gorgon) is larger, with transverse stripes of black on
the neck and shoulders.
[1913 Webster]