My only advice is...nip this in the bud NOW. I've had this problem with Alex all his life...tried many different things to make it stop, and it never did. To this day, he could be sleeping on his bed, and if any kind of animal comes on the TV, OR any kind of violence..like wrestling, he will charge the TV barking. At almost 8 years old now, I can only blame myself for not being consistent enough to make this behaviour stop. And now, the only thing that works is changing the channel.
Good Luck!!!