The Early History of Terriers
French Eching showing Earthdogs c 1560
Terriers were first classified as such by Dame Juliana Berners in 1486 AD, calling them 'Teroures'. This wonderful French etching from around 1560 AD demonstrates that for centuries certain types of scent hounds dug into tunnels and underground lairs to find and hunt prey that lived beneath the ground. As these seem to be long low-slung dogs, with deep chests, they appear to be similar to 'the 6 best earth dogges in Argyll (Scotland)' which James 6th of Scotland sent to France in 1600 AD.
A Scent Hound named 'Terrier'
Terrier Hunting with net c 1584
In 1570, Dr. Johannes Caius called the type of scent hound that follows its prey beneath the ground Terrier in his classification that was written in Latin and translated into English by A Fleming in 1576 AD. The original translation of this important work is difficult to read to be printed in its original form.
Dr Caius extract on Terriers
So it appears here as my interpretation in modern English:
'There is another sort of Scent Hound that only hunts the fox and the grey badger. These are called Terriers because they (after the manner and custom of ferrets, in searching for rabbits) creep into the ground, and frighten by nipping and biting the fox or the badger. Alternatively these terriers either tear them into pieces with their teeth in the bosom of the earth, or else haul and pull them, by force, out of their lurking angles, dark dungeons, and close caves, or at least through conceived fear, drive them out of their burrows. So the fox or badger is compelled to bolt, being desirous of the next, albeit not the safest refuge, the fox or the badger is otherwise taken and entrapped with snares and nets lain over their holes'.
Terriers Split into Regions
British Isles showing roughly the areas where Terriers Originated
By the early 1800's when the various different terrier types began to emerge, there were no formal registration records. Terriers of various shapes and sizes were born in the same litter. Huntsmen and farmers did not care what the terrier looked like as long as it did the required job. The various Terrier breeds we know today were originally developed by peasants or working-class people living in the localized areas of the British Isles roughly depicted by the accompanying map.
The Emergence of the Terrier Breeds
British Terriers of Various Types c 1906
Selection was on the principal that any dog that was adept in a particular task was mated to another adept in the same task. So we have emergence of the various breeds of Terriers. Because peasants rarely travelled outside their local region, today many terrier breeds still bear the name of that region of the British Isles where they first developed. For this reason, we shall consider Terriers according to those Regions.
The terrain in which these terriers developed is very important. Also, if the huntsman rode a horse, he would be more likely to use long legged terriers. However, if the huntsman hunted on foot, he is more likely to use short legged terriers.
References and Further Reading
 Dr John Caius, Of Englishe Dogges: The Diuersities, the Names, the Natures, and the Properties, trans. Abraham Fleming (London, 1576), 36. Originally published in Latin in 1570 as Johannes Caius, De Canibus Britannicis.
Also published in 2015 as"The Historical Function of Terriers" by Jane Harvey in 'The Talkabout" the Official Publication of the Australian Terrier Club of America 2015 Issue 2 Pages 36 - 37
See also Jane Harvey, DVD "Terriers Then & Now" (Rangeaire Vision 2002, 2004) ISBN 978-0-9804296-4-0