Put an animania chick in the pence, and wait till its time to grow up, what you will see instead is a chicken spawned outside of the block, and the chick is still inside of it, bigger than before. I think it is because when the chick grow up, it transform into a different mob, in this case it is unable to, and will keep spamming chickens while being kept inside the pence.

I will look into this, but no promises on this. Animania and my mod have had a few incompatibility issues. Animania has at least made it so my mod actually accepts them as chickens.

Edit: Yeah it looks like the chicken is set to dead in the pen, and a male or female will spawn. I can handle the chicken being set to dead.. but as far as the chicken spawning outside the pen. I cant handle that. I will have no way on knowing if that chicken is suppose to be inside the pen when it spawns, as it is a completely new entity.