You probably remember Cyberdog Rex from Fallout: New Vegas. Rex was a former police dog who was cybernetically enhanced, and while many of his parts were replaced with robotics he remained 100% A Very Good Boy. A quest in New Vegas involved finding Rex a replacement brain, as his organic one had begun to deteriorate.
While Fallout fans also love Dogmeat, it's nice to see Rex again, and you can bring him to Fallout 4 with the Project Cyberdog Rex Dogmeat Replacer mod created by Shadowliger. This is a replacer mod and is a visual change only, though the modder is apparently working on a larger Cyberdog mod, which is detailed on the mod's page:
"Project Cyberdog is a large project I am in the process of creating with the aim of allowing players to build Cyberdog armor for their dogs and customize everything from dog breed (fur patterns) to paint jobs (police, military, law enforcement, etc.) to how many robot legs/parts they want their dogs to have. There are also custom accessories made specifically for Project Cyberdog to allow players to further deck out their dogs."
In the meantime, this Dogmeat Replacer mod will do just fine, if only to see Rex again. And here are bunch of pictures that let you do just that: