Warning: some less knowledgeable will assume I am using bad language. I am not.
In days of old when nights were cold, oh, wrong story.
In the old days, most people walked to get about. There were other forms of transportation, but they were used by the wealthy and royalty. In some instances, they were exclusive to royalty. One of those modes was the ass. Some folks would prefer I use the word burro or donkey.
A really wealthy person or royalty would have a stable of said animals. If they traveled long distances, they would have a retinue. The animals would be rotated like we rotate the tires (tyres) on our vehicles to extend tread life. They would be a able to actually travel farther and more quickly by allowing the animals to rest unburdened.
If an animal were to die in 'the traces', as it were, a servant would be dispatched to get a replacement, one which had the most rest, much like a manager signalling for a relief pitcher, was brought forward. The retinue would continue, leaving the dead animal beside the 'road' to be dispatched by scavengers. The natural order of things, PETA.
The next time one hears a human being telling another human being to 'get off their dead ass...', remember, either person is likely ignorant of the origin of the phrase. Person One is simply pointing out a servant is not going to appear with another imaginary ass, donkey, burro. to carry Person Two away.
As for the 'Dead Parrot', may I direct you to the 'Egress'. Keep a sharp eye out for a 'Python' named 'Monty'.