Ernie the Chicken
Ernie the Chicken has a tenancy to randomly pop up in an episode and have an epic fight with Peter. These fights tend to go on for a long time. Ernie's first appearance is in the "Da Boom" where he angers Peter in a flashback by giving him a coupon which Peter finds out it is expired when he tries to use it, resulting in the beginning of a series of intense, bloody fights with the chicken, each usually ending a defeated rooster who'd lie still and unconscious on the ground. Each scene ends with Ernie giving twitching.
After a few fights, they forgot about why there fighting and Ernie makes it up to Peter by taking him to dinner at a fancy restaurant. When they cannot decide who will pay for the bill, another fight starts.