Looks like a meat spot. Fairly common. Occasionally things get loose inside the hen's body cavity. If it makes its way into the hen's internal egg making factory it can wind up in the egg. Blood spots are more common but meat spots happen. That one looks pretty gross.
Egg Quality Handbook
http://www.thepoultrysite.com/ourbooks/1/egg-quality-handbook/
Commercial egg laying operations candle their eggs to remove any with internal defect like that so customers don't get that kind of a surprise. If the defect isn't hat bad the eggs are sold to places that break the eggs before they use them, bakeries, pet food, things like that. That is the perfect example of why it is a good idea to break our eggs into a separate bowl before we use them, you never know what yo will find in them.
That has nothing to do with a rooster.