I have ventured to subjoin this recital from the charter of William, thinking that it may be acceptable to your querist, as fully explanatory of the transaction to which his question refers.
LAMBERT A. LARKING.
Donkey (Vol. v., pp. 78. 165.).
—In Chaucer's Canterbury Tales, v. 16954., we have—
"Ther gan our hoste to jape and to play,
And sayde: sires what? Dun is in the mire."
There is also an old proverbial simile:
"As dull as Dun in the mire."
It is supposed that Dun was a nickname applied to the ass from his colour, in the same way as Burnell, in the Chester Whitsun Playes, MS. Harl. 2013., and Russell applied to the fox, Canterbury Tales, v. 15340.
As to the termination key, it is probably (as in monkey, jockey, which are the only words of similar formation which I can call to mind at present) the same as kin, which has the force of a diminutive in words like lambkin, mannikin, &c.