Not looked up "knackered" but from memory I recall if a bull has had its testicals removed it was deemed to be "knackered"
Another meaning "of being exhausted" might be from the farmer "being exhausted" from "knackrting" a herd of bulls in a *knackers yard" to improve live stock
Had to look up the below
The
Knocker,
Knacker, or
Tommyknocker (US) is a mythical, subterranean,
gnome-like creature in
Cornish and
Devon folklore. The
Welsh counterparts are
coblynau. It is closely related to the
Irishleprechaun,
Kentish kloker and the
English and
Scottishbrownie. The Cornish described the creature as a little person 2 ft 0 in (0.61 m) tall, with a disproportionately large head, long arms, wrinkled skin, and white whiskers. It wears a tiny version of standard miner's garb and commits random mischief, such as stealing miners' unattended tools and food.