[sbierwagen]

I've got a dual dynamo/drum-brake hub, the Sturmey Archer X-FDD. It works quite well-- but only at normal biking speeds. Below 10mph it's flickery and annoying.

If a horse isn't in a hurry, it usually travels at a walk, which is something like 5mph. So bicycle dynamos won't work well for a buggy.

[maxerickson]

It could be mounted with a belt (at a reduction). The horse probably won't be as bothered by the increased load as a human.