Plutarch, Life of Cato the Elder
5.2-5 (tr. Bernadotte Perrin):
And yet we know that kindness has a wider scope than justice. Law and justice we naturally apply to men alone; but when it comes to beneficence and charity, these often flow in streams from the gentle heart, like water from a copious spring, even down to dumb beasts. A kindly man will take good care of his horses even when they are worn out with age, and of his dogs, too, not only in their puppyhood, but when their old age needs nursing.
While the Athenians were building the Parthenon, they turned loose for free and unrestricted pasturage such mules as were seen to be most persistently laborious. One of these, they say, came back to the works of its own accord, trotted along by the side of its fellows under the yoke, which were dragging the waggons up to the Acropolis, and even led the way for them, as though exhorting and inciting them on. The Athenians passed a decree that the animal be maintained at the public cost as long as it lived.
Then there were the mares of Cimon, with which he won three victories at Olympia; their graves are near the tombs of his family. Dogs also that have been close and constant companions of men, have often been buried with honour. Xanthippus, of olden time, gave the dog which swam along by the side of his trireme to Salamis, when the people were abandoning their city, honourable burial on the promontory which is called to this day Cynossema, or Dog's Mound.
We should not treat living creatures like shoes or pots and pans, casting them aside when they are bruised and worn out with service, but, if for no other reason, for the sake of practice in kindness to our fellow men, we should accustom ourselves to mildness and gentleness in our dealings with other creatures.