I was placing down the kitchen and before the walls were down a cassowary got caught inside. I thought it would have enough food brought in by the chefs to survive and not go crazy on one of my colonists. About one or two seasons later the cassowary mined through my limestone walled kitchen and escaped. I'm not sure if that behaviour was intended for the cassowary's.
It's probably a side effect of the changes Tynan made to stop people from starving trade caravans. If it was a neutral cassowary, not belonging to any faction, then it sounds like a bug. It's not like it was a sabertooth badger.