In Georgette Heyer’s The Unknown Ajax, young Richmond Darracott is mightily impressed when his rakish cousin Vincent arrives,
“…driving himself in a curricle to which were harnessed three magnificent black geldings, randem-tandem…and exclaimed: ‘Here’s my cousin at last! Oh, he’s driving unicorn! He’s the most complete hand!'”
[He asks] ‘What’s this new quirk, Vincent? ….Is it now the high kick of fashion to drive–unicorn, do you call it?’
‘Yes, or Sudden Death,’ replied Vincent…’And no, little cousin, you may not drive them. We have had enough sudden deaths in the family.’
When I first read The Unknown Ajax, there was no Google-search, and the dictionary only described the mythical horned beast, so I had to guess what the term meant.
Later I learned that driving a team of horses unicorn-style means there are 3 horses pulling the carriage — a pair of wheelers behind a single lead horse.
Farmers would hitch their horses in this fashion in order to negotiate closer turns along the edge of the field. Coachmen might drive unicorn when one horse in a team of four went lame and had to be left behind.
This difficult style of driving, also known as “randem (sic)-tandem,” requires a particularly well-trained lead horse and an experienced driver (see, e.g., Jennifer Kloester’s Georgette Heyer’s Regency World, p. 169)
Even trickier was driving a coach in a pickaxe hitch — five horses, three across at the front and two wheelers behind them. Apparently the fashionable Corinthians and Go’s wanting to show off restricted themselves to driving tandem (one horse in front of another) or unicorn, and left the pickaxe hitch to professional coachmen.