The Collaborative International Dictionary of English v.0.48:

pachyderm \pach"y*derm\ (p[a^]k"[i^]*d[~e]rm), n. [Cf. F.
   pachyderme.] (Zool.)
   Any of various nonruminant hoofed mammals having very thick
   skin, including the elephant, rhinoceros, and
   hippopotamus, one of the Pachydermata.
   [WordNet 1.5 + 1913 Webster]

WordNet (r) 3.0 (2006):

pachyderm
    n 1: any of various nonruminant hoofed mammals having very thick
         skin: elephant; rhinoceros; hippopotamus