The German Shepherd had lost control of its back legs and bladder.
The maggot-infested wound was at least a week old, vets say.
The SPCA received a complaint about the horse's condition 15 months ago, but was satisfied then the owner hadn't breached the Animal Welfare Act.
Locals complained about the horse but nothing was done.
On one trip, a skipper was noted telling crew to kill "as many as possible".
"People need to know animal abuse should not be used as a tool to get more views, likes, and shares."
A duffle bag filled with puppies dumped in a river and a dog with rotting flesh are among this year's worst offences.
They've launched a petition demanding action against animal experimentation.
A bid to prosecute Brendon Cole for allegedly live-baiting animals will not go ahead.
