Cyberdog

Cyberdog is a generic term for any dog that has been modified robotically. For some dogs, such as Robodog of NCR's Dr. Henry, the modifications are basically physical modifications, such as robotic enhancements to limbs. Others, such as K-9 of Navarro's Dr. Schreber, are more extensively modified, with enhancements to the brain and vocal organs allowing intelligence and speech.

Fallout: New Vegas
There is a cyberdog companion in Fallout: New Vegas called Rex

Rex is a fairly decent companion granting a perk which allows the PC to see items while zoomed in. The items will be highlighted green. Rex has a low DT and can be downed quickly until upgraded by replacing his brain with three other dog's brains found in the wasteland In the quest "Ain't Nothin but a Hound Dog," Dr. Henry, a character from the original Fallout who worked on cyberdogs, is the character who performs the brain surgery on Rex.

Achieved
In Fallout 2, one may be obtained at the New California Republic if you inject a super mutant of your choosing with a sort of chemical that supposedly stops genetic damage. This quest is received from a scientist, and after you report your results back to him he will reward you with Robodog.

Appearances in games
Cyberdogs appear in Fallout 2 and Fallout: New Vegas.