That’s an awesome idea! I love it.
I did find a couple on Etsy, but they were pretty big (1:6 scale), and super expensive. You might try contacting one of those sellers to see if they can do a smaller version one-off special for you.
Or, you could find someone who knows how to convert 3D models into STL files. Most of the unofficial Fallout minis I’ve seen were just ripped from the game files, which are super easy to get, so it wouldn’t be too difficult to convert it to an STL, I would think.
Alternatively, you could look up “Clockwork Horse” and make do with some clever painting and detailing–as well as clever storytelling–with some of those models
Story-wise, I could come up with a bunch of ideas. Have your party really work for it by saying they found parts to a special concept model Wilson Atomatoys used in live demos that was waaay too complex to market and make affordable (“affordable” in Fallout terms, anyway").
Run it with a Boston Dynamics parallel. Maybe it even moves similarly to Spot. For an in-universe explanation as to why it was never mass produced, the gyroscope thingamabob (insert technobabble) was so advanced on this Buttercup model, which Arlen designed after a real horse, of course (of course) (SorryNotSorry). Wilson Atomatoys sold the patent to RobCo, which then used the gyroscope to put in their Assaultrons so it didn’t keep tripping over itself and explode.
So, your party isn’t just building a standard Giddyup Buttercup, so now they have to go out and find robot parts from other robots to get this one to run, because of course RobCo found a way to backward engineer and weaponize all the Buttercup tech without Wilson’s (and ostensibly Arlen’s) knowledge to use in their other robot models. The Automatron DLC allows for such a mash-up of weird robots, I don’t see this as being too far out there.
Meanwhile, Arlen had to go back to the drawing board and super-streamline his original design for a mass produced commercial product. It’s not really clear in the lore, but I always assumed Giddyup Buttercups weren’t really robots so much as remote control?
Granted, that’s a long way to go to use a $10 D&D mini, and your group would have to suspend disbelief a bit–okay, a lot–but to me, that’s the fun part of role-playing. In fact, I might use this idea myself!