From the "Animal Rights History" website:
1903 | A Dog's Tale
The laboratory was not a book, or a picture, or a place to wash your hands in, as the college president's dog said—no, that is the lavatory; the laboratory is quite different, and is filled with jars, and bottles, and electrics, and wires, and strange machines; and every week other scientists came there and sat in the place, and used the machines, and discussed, and made what they called experiments and discoveries.…
…They discussed optics, as they called it, and whether a certain injury to the brain would produce blindness or not, but they could not agree about it, and said they must test it by experiment.…
…Suddenly the puppy shrieked…I ran at once to my little darling, and snuggled close to it where it lay, and licked the blood, and it put its head against mine, whimpering softly, and I knew in my heart it was a comfort to it in its pain and trouble to feel its mother's touch, though it could not see me. Then it dropped down, presently, and its little velvet nose rested upon the floor, and it was still, and did not move any more.
(Mark Twain, A Dog's Tale 
1906 | A Horse's Tale
How many times have I changed hands? I think it is twelve times—I cannot remember; and each time it was down a step lower, and each time I got a harder master. They have been cruel, every one; they have worked me night and day in degraded employments, and beaten me; they have fed me ill, and some days not at all. And so I am but bones, now, with a rough and frowsy skin humped and cornered upon my shrunken body—that skin which was once so glossy, that skin which she loved to stroke with her hand. I was the pride of the mountains and the Great Plains; now I am a scarecrow and despised. These piteous wrecks that are my comrades here say we have reached the bottom of the scale, the final humiliation; they say that when a horse is no longer worth the weeds and discarded rubbish they feed to him, they sell him to the bull-ring for a glass of brandy, to make sport for the people and perish for their pleasure.
(Mark Twain, A Horse's Tale  "Solider-Boy—to Himself")