Animals have most likely been stuffed since man or
his ancestors began cooking food. After cleaning out the body cavity
of internal organs, you have a hollow space that just begs to be
filled. Especially for birds, rabbits, hare, suckling pigs, etc.
They have a small cavity, and the stuffed animal could be cooked
through in a reasonable amount of time.
The ancestors of our modern chickens were first domesticated in
western Asia, in the area around India, probably about 4,000 or so
years ago. Stuffed chicken most likely followed soon after.
The oldest cookbook that we have is by Apicius, a Roman gourmet who
lived some time around the 2nd century BC to the 1st century AD. In
his cookbook 'Apicius de re Coquinaria', he has recipes for
stuffed chicken, hare, pig, and even stuffed dormouse. Most of these
early stuffings were of vegetables, herbs, nuts, spelt (an ancient
variety of wheat) and frequently chopped liver, brains, etc.