Walter Map, writing around 1190, tells the story of King Herla, whom he knew as its leader, and later adds:

"The nocturnal companies and squadrons, too, which were called of Herlethingus, were sufficiently well-known appearances in England down to the time of Henry II, our present lord. They were troops engaged in endless wandering, in an aimless round, keeping an awe struck silence, and in them many persons were seen alive who were known to have died. This household of Herlethingus was last seen in the marches of Wales and Hereford in the first year of the reign of Henry II, about noonday: they travelled as we do, with carts and sumpter horses, pack-saddles and panniers, hawks and hounds, and a concourse of men and women. Those who saw them first raised the whole country against them with horns and shouts, and . . . because they were unable to wring a word from them by addressing them, made ready to extort an answer with their weapons. They, however, rose up into the air and vanished on a sudden."

This is a passage from "Albion - A guide to legendary Britain, Jennifer Westwood, Granada Publishing, London 1985

Walter White mentions this term in "Notes and Queries" also

Basically a "Herlethingus" was the Middle English term that became an Old French term "hellequin" used by by the Norman monk Oderic Vitalis, who used the word to describe a group of bandits who attacked him in Normandy in the 11th century... separating it from its association with King Herla and coming to mean a band of unruly demons generally.

Essentially the word was originally a pack of specific "Wild Hunters" associated with the soldiers of King Herla... which came later to mean a pack of demons generally. A "Harlequin" is therefore a representation of the devil (or a devil).

It is a particularly apt description to call a Rugby team a familia herlethingi.

I think the truth is that it's a disambiguous term that has been adapted by various people over the years to suit their own needs. In the case of the Harlequins rugby club, they wear a multi-coloured kit to echo their name in the theatrical sense but I think they'd much rather be described in the classical sense /u/zyzzogeton outlined :) The progenitor of the modern Harlequin is the character Harlequin from the Commedia dell'Arte, a sort of Italian pantomine.

In modern theatre, the Harlequin is often an MC or agent provocateur, who's role is... not quite a fool. There's a playful aspect to them that conceals a bitter truth or acerbic point. While the classical fool is about slapstick blunt humour, the harlequin will often use those same techniques to conceal a much sharper point. I think the phrase I am hunting for is "Tragic Comedy".

For example, in "Oh! What a lovely War!" the MC, often portrayed as a Harlequin, the only splash of colour amongst the white-uniformed clowns who portray the soldiers, is often seen playfully interacting with the soldiers - pushing them over, laying them down to sleep, handing them poppies, ushering them off stage - but the deeper meaning to the slapstick interaction is that each of these actions represents the soldiers death. This interaction between the clowns in their Pierrot costumes and the Harlequin also echoes the classical origin of the character Harlequin in Commedia dell'Arte.

An example of a classic "Oh! What a lovely War!" production: http://www.queens-theatre.co.uk/archive/2002/ohwhatalovelywarcompany.jpg

Pierrot clown and Harlequin by Cézanne: http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Paul_C%C3%A9zanne_060.jpg/478px-Paul_C%C3%A9zanne_060.jpg

I would thoroughly recommend looking up Richard Attenborough's adaption of the play on youtube. This is a great example of the Harlequins role:

https://www.youtube.com/watch?v=zr5ksOyxZRU

The chap in black is the one who'd be the Harlequin on stage. Note his role - he instigates the song, gets the soldiers moving, drives the train, hands out the poppies... he's driving the action without it appearing that he does.

Sorry if I'm waffling, it's a difficult concept to express :)

Incidentally the female rugby teams are named after leading rugby club Harlequins, who originally formed as Hamsptead Football Club (Rugby being a variant of football in case anyone was wondering) in 1866. In 1869, not only were they forced to move from Hampstead, their membership was drawn from further afield so a name change was decided.

Apocryphally they were called Harlequins at the suggestion of one of their members simply because it was a frivolous name that kept the HFC identity that was monogrammed on their shirts.

The Harlequins: 125 Years of Rugby Football. by Philip Warner

My favorite use of the term is in Harlan Ellison's short story, "Repent Harlequin, said the tick-tock man"

In this story, the "harlequin" is a whimsical-jester type character - who disregards conventions (in this case timeliness at all costs).

A great read, if you're into thought-provoking, well-written short stories.