"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?
How'd it go for me at the blackjack tables last night? I got pounded like a rented gerbil, that's how! I dropped almost two grand! Fuck... that'll teach me to gamble with money I can't afford to lose.