Times of technological transition make interesting things. Because of the transitional nature of the things, they're usually not destined to be around long, but they're absolutely necessary for a certain magical moment. Think 8-track to cassette adapters or those USB 3.5" floppy drives they had for the first iMacs.
One of the biggest technological transitions in human history was the move from animal-powered to motorized vehicles. The reasons why are well-understood (humanity was sick of staring at horse anuses all the damn time), but where are the missing links between horse carriage and car?
While many early cars were certainly adapted horse carriages, you would think there would have been some sort of stopgap solution for all those carriages that were in use– an adapter of sorts. There were, but there's surprisingly little information about them available online. In fact, I first learned of them through a book, like an animal would.
The book was Floyd Clymer's 1957 Treasury of Foreign Cars , and in that book I learned about the 1899 Kuhlstein-Vollmer Motor Vorspann, which was pretty much exactly what you'd imagine a machine to transition between automobile and horse would be: a motor with two wheels that connects to a carriage in place of a horse. I'd always wondered if something like this existed, and it appears it did. The controls were accessible to the carriage driver, in roughly the same location as where the reins would be. From what I can tell, there's a steering wheel and a secondary wheel with levers that I suspect is for throttle and possibly spark advance, and perhaps even fuel mixture control.
With units like these, existing fleets of carriages and carts could be kept in service. In the book, it's mentioned that the German Empire's postal service used these with their delivery wagons, to great cost-savings. I'm not really sure how braking worked– I suppose that was the responsibility of the existing carriage, or the driver's skill.
A bit of research shows that our side of the pond had similar solutions. In a big automobile encyclopedia I saw an entry for the 1899 International Motor Wheel, which seems to serve pretty much the same horse-replacing role as the Motor Vorspann. The International Motor Wheel used just one wheel, with an engine and flywheel flanking it, like the bread on a motor wheel sandwich. Like the Vorspann, this was meant to replace a horse, and as such the scale of it is pretty intimidating– taller than an average man. It's a nice study in simplicity, especially compared to the two-wheel German model. One wheel makes steering very easy, and you can reverse by just turning it all the way around! Clever.
It looks like there were other motor wheels out there at the time, like this Briggs and Stratton model, but they were designed to be mounted to the rear of a vehicle to push, and were not really horse replacements. Still, I've imagined just such a device many times while stranded by the side of the road. A nice little motorized wheel I could clamp to my rear bumper to get my ass home. It'd be like having your own tow truck!
I wasn't able to find how popular these devices were. I suspect they had a core following, but in the end most people decided to go all the way and move to an actual car as soon as they could. I was trying to think of other, more recent situations that mirror this, and I realized we do have one very strange parallel: the switch to HDTV.
When HDTV replaced analog broadcasts a few years ago, consumers had a couple options: buy a nice new, not-nearly-as-awful-to-move flat screen television, or buy a converter box for your existing CRT TV. The converter boxes let your old TV work with the new format. It's doing essentially the same thing, but in a modernized way. This is exactly what the motorized horse-replacing wheels were doing– buying some extra life for your carriages, letting them do essentially what they did before, but motivated by a new, modern force.
And, like these mechanical horse-replacers and motor wheels, I suspect the number of people who convert their older technology to survive in the modern world is relatively small. I only know a few people who still have CRT televisions, the math of money savings to benefits clearly favoring a new flat screen. I imagine something similar happening at the turn of the last century, as the performance and use of these mechanical horses likely didn't match even the limited purpose-built automobiles of the time.
Still, these equine impostors do have a certain charm, and it's interesting to think about how things could have been different if they had been wildly popular– perhaps our auto industry would have been divided into separate motor unit and coachwork companies. You may own one nice Mitsubishi V6 traction unit (with integrated LED lights and a wireless instrument pod) that can connect to your small Pininfarina open top roadster carriage, or your German 6-passenger Karmann sedan body (with Nav system and great leather seats), or your useful American Fisher cab-and-truck, with a great stereo and a bed that can haul full-size drywall sheets.
Plus, I bet YouTube would be full of morons hooning the traction units alone while standing on them, like a 200 HP Segway.