I use the word functional instead of usable or controllable because that’s kind of under the umbrella of how they could be implemented.
Both Fallout 3 and Fallout 4 had rail systems you could see moving (the presidential metro and Nuka-Express) opposed to Fallout New Vegas (Camp McCarran’s monorail) for example.
I feel like the two major concerns would be how their addition would effect map design and how the engine is going to handle them.
I wonder how many horses would still be around or what other mounts they could go with?
I believe the appearances of horses in the Fallout New Vegas comic was stated to be an error and some of the early games mention they were wiped out.
There is obviously a large number of animals that survived but mutated in various ways. Lack of regional communication could explain their introduction.
Brahmins show that farm animals did survive, so it wouldn’t be much of a stretch that horses survived in the Rockies and nobody outside of that area knew they existed.
Of course what I was saying though was that I thought horses were canonically confirmed to not exist but it seems like people were misquoting a post on Fallout fandom.
How about a sabre toothed donkey?