In the very beginning of chapter 1 (when you have to fight the Lambent to get to the CIC room), after you kill all the enemies, go to your left, and you'll see 2 pipes.

Go very close to them, and try to shot at a hole inside it (you'll know when you took enough shots after Marcus says: "Hellooo!!!").

After that, shoot inside the other pipe, turn around and there'll be another 2 pipes, shoot the one in the right and finally, the one in the left.

A little chicken will jump out of the last pipe! Shoot it VERY slowly (try it with the Snub Pistol) and this little chicken will turn into a Giant Fire Breathing Chicken!

It is a little hard to kill, but gives you lots of XP!