The Collaborative International Dictionary of English v.0.48:

impala \im*pa"la\, n.
   An antelope (Aepyceros melampus) of Southeastern Africa,
   the male of which has ringed lyre-shaped horns, which curve
   first backward, then sideways, then upwards. ALso called
   impalla and pallah.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Impalla \Im*pal"la\, n. (Zool.)
   Same as Impala.
   [1913 Webster]