A unicorn is a driving configuration with two horses behind and one in front. It may be the most difficult configuration to drive and requires a very special lead horse (if you look at ads for driving horses, if the horse has experience leading a unicorn it will be mentioned).
It's also called "randem tandem" and was often used by farmers who needed to make tight turns - or if a coach needed to go somewhere and one member of the four-in-hand was lame. In some periods, also, driving unicorn was a way of showing off one's skill with the lines.
This unicorn rig is being inspected by a judge after completing an obstacle course (or before starting, I can't be sure). Unicorns are, these days, mostly seen in competition - as a way of showing off one's skill with the lines, of course. Image source Eponimm via Wikimedia Commons.