Old Ulysses is a giant fish that inhabited the waters of Possum Lake.
In "The Legend of Old Ulysses", Stan leads the entire family to Possum Lake for a traditional getaway. Klaus, who tagged along uninvited is promptly swallowed by Old Ulysses when he tries to take a swim. When Steve and Stan find themselves losing contests at the camp to Snot and Tuttle, with whom he hooked up to be his surrogate dad, Steve breaks them up. But when Stan expresses regret for the dirty tricks, Steve ditches him to become Tuttle's partner for the Possum Lake boat regatta.
With Snot out of the race and Stan struggling to sail his ship alone, Steve starts to feel regrets that he may have hurt two people close to him. But things go crazy when Old Ulysses decides to attack the regatta, wrecking boats and swallowing Tuttle in the process. With Stan knocked out cold from one of the fish's attacks, Steve is able to revive him with Tuttle's stash of energy drinks. Together, they fashion a hook out of an anchor and managed to land Old Ulysses, freeing Klaus and Tuttle.
The fish and his theme song make a cameo in "The Pleasanting At Smith House" when a vengeful wraith considers feeding Klaus and Jeff to him.