WordNet (r) 3.0 (2006):

fat cat
    n 1: a wealthy and privileged person

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "fat cat":
   Daddy Warbucks, big cheese, big chief, big shot, big-timer,
   billionaire, bloated plutocrat, capitalist, heavyweight,
   man of means, man of substance, man of wealth, millionaire,
   moneybags, moneyed man, muck, mugwump, multimillionaire, nabob,
   parvenu, plutocrat, rich man, richling, warm man, wealthy man