Rabid Dogs are monsters in the facility. They are not listed on the whiteboard. They can briefly be seen and heard barking in their cube prison during the scene when Marty and Dana are in the elevator.
They are shown briefly during The Purge, mauling a facility worker on a monitor in the control room.
A better view of them can be seen in the special features on the DVD.
Tampering with the dog whistle summons them.
Vicious, killer dogs have been used in many film and stories over the years. They are inspired by the behavior of wolves, feral dogs, and rabid dogs, which stems from Cynophobia. Examples include Cujo and The Breed and many others.