"Pounded like a rented
gerbil" is a phrase used to describe disastrous events- usually painful financial setbacks.
As you can
imagine, rented gerbils would be the unfortunate recipients of an especially rough and vigorous pounding experience, when compared with the more tender romantic encounters of privately owned gerbils. After all, you'll be returning the
poor rodent right back to the rental office tomorrow, so why bother taking good care of it?