plutarch on the care of dogs and other beasts

Kindness indeed is of wider application than mere justice, for we naturally
treat men alone according to justice and the laws, while kindness and gratitude,
as though from a plenteous spring, often extend even to irrational animals.
It is right for man to feed horses which have been worn out in his service,
and not merely to train dogs when they are young, but to take care of them when
they are old. . . .
Many persons, too, have made friends and companions of dogs, as did
Xanthippus in old times, whose dog swam all the way to Salamis beside his
master's ship when the Athenians left their city, and which he buried on the
promontory which to this day is called the Dog's Tomb.
We ought not to treat living things as we do our clothes and our shoes, and
throw them away after we have worn them out; but we ought to accustom ourselves
to show kindness in these cases, if only in order to teach ourselves the duty
towards one another. For my own part, I would not even sell an ox that had
laboured for me because he was old. . . .

From Plutarch's Life of Cato, V. Plutarch was a Greek historian living shortly after the lifetime of Christ -- 46 to 120 AD -- famed for his biography series of famous Greeks and Romans.