creatures that cannot be attacked

I came across creatures that cannot be attacked. They have no life bar. They don't respond to aoe attacks. However, they can attack themselves and react to collisions with objects.

I can be seen in the video from minute 1.

https://www.youtube.com/watch?v=v5zPaWz5bRM

OK, I checked a little. Mobs appeared with "imprisoned monster" but before clicking.

After clicking and releasing the mobs, they could be easily killed.