Donnie is an ex-boyfriend of Penny.
He is mentioned in "The Boyfriend Complexity" when Penny's father Wyatt tells Leonard about Penny's ex-boyfriends. Donnie is mentioned as a genius and a "rocket scientist" who was gonna make millions turning farm waste into biofuel and selling it to the government. All Donnie did was mix pig poop with a little water and pump it into his mom’s Camry. Donnie was a rocket scientist compared to a boy named Curtis who wanted to get beer pong into the Olympics.