Knew a guy in West "By Gawd" Virginia a decade or so ago who had one attached to his hunting Vee Dubya (Classic 68). He put two dog cages on the trailer and wheeled his beagles all over the state to rabbit hunt.
Neat trailer, as you said it was attached to the rear bumper and the single wheel was (in effect) a castor. It was so much of his car, the car plates were on the trailer and I don't think it was licensed as a trailer. Sure attracted a lot of attention.
BTW there are motorcycle trailers that have one wheel. One brand is
uni-go
Rap