That has to be one of the best video game easter eggs I have ever seen.
In chapter 4 right before the end of the chapter when you're driving in the car to the farm, just drive right by that farm and follow the road as far as it can go. When you reach the end of the area you can drive in, hop out and continue along the road going past all the obstacles until you spot a field to the left of the road. Hop off the road and in the field look to the left of the field for a House and a Windmill. Behind the House is three graves with wooden crosses across them. Approach the graves and you trigger a hidden battle against three Evil Dead styled skeleton enemies that are different than every other enemy in the game, they chase you around but will split apart and throw their body parts at you, but after thrown they will come back and reassemble back into a skeleton.
The Skeleton's are animated perfectly and to get an idea, they look and are animated like this: • Jason And The Argonauts - Skeleton Fight
This easter egg is freaking amazing, I wonder if there are more huge ones like this...