I came across these some time ago. They are manufactured in the Schloss at Kühlenfels in Oberfranken, Germany. Introduced in about 2005, they are still in production, as far as I know. For those wanting something a bit more sporting, there is also a smaller two-seater model called the "Duc", shown below.
These are not replicas of anything in particular, but are recreations of (supposedly) typical horseless carriages of the 1890s. And for the authentic horsedrawn experience, the Aaglanders are steered by two stiffened leather "reins", which control hydraulically-operated power steering, giving the driver the feeling of controlling a horse (and onlookers much amusement, no doubt).
They don't give many details about the engines, but early models had 719-cc 3-cylinder diesel units, which I assume would be Kubotas. As Iluvatar says, they currently appear to ship with 900-cc units, to provide a bit of extra poke.
This is coupled to a continuously variable automatic transmission, driving the rear wheels by heavy chains. And if this arouses your "boy racer" instincts, there are dual-circuit hydraulic disc brakes all round to pull you up smartly. There's no mention of ABS or traction control, so it's probably best to exercise a bit of caution if exploiting the machine's top speed of 20 km/h.
There's no problem if night falls, as the reproduction oil lamps conceal halogen headlights, and should you become totally carried away with the freedom of the open road, and find yourself hopelessly lost, don't worry, as you can specify an onboard GPS system to guide you safely home.
As for the price, I haven't been able to find out, but I suspect that if you have to ask, then you probably can't afford it.
So what to make of it? Magnificent, or mad? Inspirational, or utterly bonkers? I won't say what I think, but you can probably guess.
But overall a fine addition to that niche market of pointless, expensive toys for wealthy people to waste their money on.