It’s likely that the earliest description of a Harrier appeared in, “The Chace” by William Somerville written in 1735:

See there with count’nance blithe,
And with a courtly grin, the fawning hound
Salutes thee cow’ring, his wide op’ning nose
Upward he curls, and his large sloe-black eyes
Melt in soft blandishments and humble joy;
His glossy skin, or yellow-pied, or blue,
In lights or shades by Nature’s pencil drawn,
Reflects the various tints; his ears and legs,
Fleckt here and there, in gay enamel’d pride,
Rival the speckled pard; his rush-grown tail
O’er his broad back bends in an ample arch;
On shoulders clean, upright and firm he stands;
His round cat-foot, straight hams, and wide-spread thighs,
And his low-dropping chest, confess his speed,
Or far extended plain; in ev’ry part
So well proportion’d, that the nicer skill
Of Phidas himself can’t blame thy choice.

He knew of what he wrote. Somerville had Harriers, himself, and devoted most of his leisure time to hunting with them.  As mentioned earlier, it’s believed that the prose above was the first written description of a Harrier.