1.  packrat(n = noun.person) - a collector of miscellaneous useless objects;
is a kind of aggregator, collector

2.  packrat(n = noun.animal) bushytail woodrat, neotoma cinerea, pack rat, trade rat - any of several bushy-tailed rodents of the genus Neotoma of western North America; hoards food and other objects;
is a kind of wood-rat, wood rat
is a member of genus neotoma, neotoma
has particulars: dusky-footed woodrat, neotoma fuscipes

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. [WordNet 1.5]

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