While it was actually a little difficult to find a definitive answer to this question, I believe it was Patricia O’Neill. She’d rescued a chimp, Kalu, in Zaire in the 1980’s which actually caused problems in her marriage. Her husband, Frank, disliked Kalu’s habits of smoking his cigarettes and drinking his beer. Kalu inherited a whopping $80 million from the estate.
Closer to home, Leona Helmsley left $12 million to her pet while cutting out of her will completely several of her grandchildren.