Meanings of human swayback in English

We walked in the gardens of the governor's palace, through swayback palms and feathery tamarisks.

Looks like a swayback mule in a clown suit.

He cut lines in the earth with an old till, a swayback mule, and a horse named Jim.

A few battered cars waited on the strip of pavement fronting the rectangle, like swayback horses tied outside a saloon.

Folk shoe their horses badly, saddle them improperly, feed them poorly, then complain that they were sold a half-lame, swayback, ill-tempered hack.