The word Quorn is a proprietary name for a type of textured mycoprotein (i.e. protein derived from fungi) used as a meat substitute.

This name was first registered as a trademark—for certain edible products other than meat substitutes—by the Quorn Specialities Company of Leicester, England, in 1914. The mark consists of the words The Quorn in a figure showing a foxhound, an allusion to the Quorn Hunt (Leicester is only a few miles from the village of Quorndon, formerly Quorn, from which the hunt took its name). The meat substitute was subsequently developed by the successors of this company. (source: Oxford English Dictionary – 3^rd edition, 2008)

The earliest instance of Quorn in the sense of mycoprotein used as a meat substitute that I have found is from Super food from mushrooms, published in the Nanaimo Daily Free Press (Nanaimo, British Columbia, Canada) of Saturday 23^rd August 1986:

Quebec (CP) — The idea of sitting down to a hearty filet of fungus may not sound appealing, but British consumers have developed a taste for a new product that tastes like meat and is produced from mushrooms.
The product, called quorn, is made to look, taste and feel exactly like beef or chicken. The bio-technological process can be altered to produce different textures for different meats.
It has been sold in 80 British stores for the last 18 months.
Demand is outstripping supply, said Gerald Solomons, a researcher who helped develop quorn.
Mr. Solomons, interviewed this week at a four-day international symposium on bio-technology and food, said researchers were guided by one overriding principle in developing quorn — it had to be delicious and attractive.
“I would defy you to tell the difference between quorn and high-quality tender beef or Grade A chicken,” Mr. Solomons said. “Our chicken product is exactly like chicken except, of course, that it isn’t chicken chemically.”
Quorn is high in nutritional value and because yields are spectacular, it holds great promise for feeding the world’s hungry, Mr. Solomons said.
Quorn starts as microscopic mushrooms grown in factories. It is processed with food-quality carbohydrate grains using special fermentation techniques, then flavored, colored and packaged.
The product is rich in the same high-quality protein found in milk, has lots of fibre, no cholesterol and little fat. Preliminary data from tests on volunteers indicates it also keeps the body’s cholesterol level down.
Mr. Solomons and a team of 120 researches [sic] spent more than 15 years and millions of dollars developing the product and testing it for safety.
Mass production for the British market will be developed over the next two years and the U.S. Food and Drug Administration now is testing quorn for market in that country. There are no immediate plans to market it in Canada.
Most quorn now is sold in the form of “meat pies” and sits on supermarket shelves beside real beef and pork pies.
Initially it was cheaper than equivalent meat products but British supermarkets now charge slightly more for quorn. Mr. Solomons said quorn will be cheaper than meat when produced on a mass scale.
Starting with one kilogram of carbohydrate grain as feed, it takes three weeks to produce 49 grams of protein in chicken, six months to produce 14 grams in beef cattle and six hours to produce 114 grams in quorn.

the Quorn Hunt

Usually preceded by the, Quorn, also more fully Quorn Hunt, is the name of a celebrated hunt that meets at Quorndon, formerly Quorn, a village in Leicestershire, and in the surrounding area of Leicestershire and Nottinghamshire. The Quorn Hunt dates its foundation from 1696; its dogs were kennelled at Quorn from 1753 to 1904. (source: Oxford English Dictionary – 3^rd edition, 2008)

The earliest recorded uses of Quorn in the sense of this hunt are from two advertisements placed in the fourth edition (London, 1802) of Thoughts upon hunting: in a series of familiar letters to a friend, by Peter Beckford (1740-1811), dog breeder and writer on hunting (in fact, the book was specially reprinted for the advertisers, Messrs. Turnbull & Asser, 71 & 72 Jermyn Street, London).

first advertisement:

THE “QUORN” SCARF
The celebrated “QUORN” Hunting Scarf was first patented and introduced
by Messrs. Turnbull & Asser, and, in spite of many imitations, it
still retains its hold on popular favour. They attribute
this to the fact that the Scarf, whilst being neat
and smart in appearance, has no unnecessary
complications and is perfectly easy
and simple to adjust

It has from time to time been improved in accordance with suggestions
made by practical and experienced riders, and is thus in all
respects the most perfect Scarf for Hunting and
Riding that has yet been invented

second advertisement:

TURNBULL & ASSER’S “QUORN” HUNTING GLOVES
are well known amongst sportsmen, and are justly celebrated for their
weather-resisting qualities. They are made of a specially selected
and thoroughly seasoned tanned leather which is admir-
ably adapted for hard wear and for throwing
off water in wet weather

Messrs. Turnbull & Asser make a speciality of Stout Chamois
Leather Gloves for riding and hunting

The second-earliest instance of Quorn that I have found is from an advertisement published in the Nottingham Journal (Nottingham, Nottinghamshire) of Saturday 12^th October 1811:

TO BE LET,

For a Term of Years, and may be entered upon immediately,
A MODERN Brick-built SASHED HOUSE, situate at Hathern, three Miles from Loughborough, at a pleasant distance from the Turnpike Road leading from Loughborough to Derby; comprising Parlour, House-place, Kitchen, Pantry, two Cellars, Wash-house, &c. on the Ground Floor; three Chambers; three Attics; with suitable Garden, Orchard, Barn, Stable, and other Out Offices, and three Acres of rich Pasture Land adjoining; the whole well adapted for a small genteel Private Family, or a Gentleman Sportsman, being only five Miles from the Quorn Hunt.
For Particulars, and to treat for the same, apply to Mr. Nicholas Smith, upon the Premises.