As Mekorig pointed out, it's a member of the rodent family, but being of South American origin it's most like a member of the Family Cavidae, which includes Capabara, and the Guinea Pig (of which I personally have five). A couple years ago they found remains of another giant rodent called Phoberomys Pattersoni, it was the size of a cow. I'm attempting to put a link to the article, but I don't know if it will work, since I've never done it before.
Okay, guess the link didn't work, so I'll just copy and paste:
http://www.sciam.com/article.cfm?chanID=sa...05980A84189EEDF