The Collaborative International Dictionary of English v.0.48:

packrat \packrat\, pack rat \pack rat\n.
   1. Any of several bushy-tailed rodents of the genus Neotoma
      of western North America, especially Neotoma cinerea,
      which hoard food and other objects in their nests.

   Syn: trade rat, bushytail woodrat, Neotoma cinerea.
        [WordNet 1.5]

   2. A person who habitually saves items, even those unlikely
      to be useful. [Informal]
      [PJC]

WordNet (r) 3.0 (2006):

Neotoma cinerea
    n 1: any of several bushy-tailed rodents of the genus Neotoma of
         western North America; hoards food and other objects [syn:
         packrat, pack rat, trade rat, bushytail woodrat,
         Neotoma cinerea]