Colt-evil Definition

noun
1722, William Gibson, The Farrier's New Guide, 3rd edition, chapter XLV:
The Colt-evil is a continued Stiffness in a Horse's Yard, and is so call'd, because it is a Disease incident to Colts, and is brought upon them by having full Liberty with Mares while they are not able to cover them […].
Wiktionary
1838, R Bowers, A Modern and Practical Treatise on the Diseases of Horses.
The Colt Evil chiefly affects young stoned colts, which have full liberty with mares before they are able to cover them.
Wiktionary