A cockentrice is a very fancy food item that was popular during the Renaissance period. It's made by taking the body of a pig and the head of a rooster and stitching them together to make one creature.
Imagine taking a pig's body and a rooster's head and using a big needle and thread to sew them together. Then, you would roast or bake the creature and serve it up as a fancy meal.
It was a popular dish in the past because it looked impressive and showed off the cooking skills of the person making it. However, nowadays it's not really something that people make or eat anymore.