You're first introduced to the Monobeasts in the 2nd campaign hospital - Lower Bullocks.
They look like hairy worms that crawl on the floor, going from hiding spot to hiding spot.Killing them is also tied to one of the 2nd star objectives in "Lower Bullocks".
You will have to zoom in to be able to shoot them. The cursor will change to iron sights as you hover above them. Shoot them with
. The remains will then be cleaned by a janitor.
If they got away and hid under something, just pick it up and they should again run away from you until they find the next hiding spot. Preferred hiding spots are Snack/Drink Machines as well as Arcade Machines.Note from the comments:
You have to be in normal speed. If you have slow mode on you can't shoot them. (Thanks to ScorchPSO)