"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?