Im interested in how exactly you did this. Firstly, in the second picture you had no less than 10 ulbernauts in the town center. To lead this many back would take a very long time and they would have probablly been killed one by one as you brought them back.
Secondly, i myself tried this same thing a month ago, with a variety of creatures. Creatures cannot cross sector boundaries, which means that the furthest you can get them is just past the bend in the road that takes you out of the forest and into the plains. (Theres a sector boundary here but you dont usually notice it.) Unless the devs have changed this recently, which i highly doubt, you couldnt have done this as you claimed.
The logical explanation is that you somehow convinced a dev to spawn ulbernaults in the Town Square, which explains why there were so many.