Adult trailer?
 

Does anyone build a bike trailer for adults? As a child we had a trailer for our horses. It was very light, and handled 2 adults, with 2-27" wheels. I think something similar could be pulled by a bike. I was trying to google it, but if they do exist, don't know what they are called. Obviously this wouldn't be a high speed or long distance device, but just for fun.

look here at this link >>http://www.workbike.org/makers/index.html

I think what you are looking for is a bicycle rickshaw. In my experience, any sort of articulated trailer gets unstable if the load is more than half the weight of the pedaler. Rickshaws are pretty common in New York in Central Park; the ones I've seen look like they attach to a regular bike with the rear wheel removed, most commonly a mountain bike. I seem to recall reading that they cost about $2,000.

Well, the little cart you had for your horse is called a "sulky." I don't know if they can be adapted to hitch behind a bike, but that's probably the term you'd need to Google. It's an interesting idea. Even if you weren't giving a passenger a ride, you could probably tote some serious cargo!