The military cyber dog is immune to the Animal Friend perk, has a sonic bark used as a weapon, and has a chance to knock down the player character with its melee attack. They also appear to have the ability to self heal/repair themselves, similar to the Mk II Securitrons.
A slightly weaker variety of the military cyberdog, police cyberdogs may contain several types of small energy cells on death. They can be distinguished from military cyberdogs by their unique black and white paint, as well as the red and blue reflective patches on their backs. Notably, they appear in a class room in the X-8 testing facility during the Old World Blues quest X-8 Data Retrieval Test.
The cyberdog is a non-militarized version of the police/military cyberdog. It was more than likely used for companionship rather than combat, as it relies on its speed and bite strength alone for offensive power.
Only one instance of the civilian cyberdog exists. It is visually identical to Rex and is explicitly identified as a test version in its Editor ID ("NVDLC03CyberDog01TEST"). It does not appear anywhere in Old World Blues.
The police and military cyberdogs are variants appearing in Old World Blues.
In Old World Blues, a police cyberdog named Roxie can be created at the X-8 research center (by splicing a robot and a dog together), becoming a temporary companion for the Courier while inside the facility. She will not follow the Courier outside, waiting instead near the entrance door. If she dies, Roxie can be "revived" by creating another cyberdog.
The cyberdogs in Old World Blues are unaffected by the Animal Friend perk.
pc The mesh file for the braincase used by military cyberdogs suffers from a flaw which renders it untextured and invisible, rather than a glowing yellow to compliment the glowing green and glowing red seen on normal and police cyberdogs, respectively. This leaves the military cyberdog's brain apparently floating in mid-air with no obvious means of containment. [verified]