“As you read these stories you will see that whenever I was properly humble and willing to let something besides a human be my instructor, these various four-legged, six-legged and no-legged fellows shared priceless wisdom with me. They taught me that perfect understanding and perfect co-operation between the human and all other forms of life is unfailing whenever the human really does his required part.”

Variant: All animals are equal, but some animals are more equal than others.
Source: Animal Farm

Variant: I hate my body. I like so many other people's bodies. I like legs — a good pair of legs on someone else always makes me jealous.
Source: "Is Kate turning into Keira?" by Clemmie Moodie The Standard (1 December 2005) https://www.standard.co.uk/showbiz/is-kate-turning-into-keira-7250578.html
Source: Celebrity W.T.F’s Volume 107(16 December 2005) https://lindagallacher.blogspot.com/2005/12/celebrity-wtfs-volume-107.html

Source: Animal Farm

Source: Death's Mistress

His collected works contain no riddle about dog legs, but George W. Julian recounts Lincoln using a similar story about a calf in Reminiscences of Abraham Lincoln by distinguished men of his time (1909), p. 241: "There are strong reasons for saying that he doubted his right to emancipate under the war power, and he doubtless meant what he said when he compared an Executive order to that effect to 'the Pope’s Bull against the comet.' In discussing the question, he used to liken the case to that of the boy who, when asked how many legs his calf would have if he called its tail a leg, replied, 'Five,' to which the prompt response was made that calling the tail a leg would not make it a leg."
A very similar riddle about cow legs was also circulated by Edward Josiah Stearns' Notes on Uncle Tom's Cabin (1853), p. 46: '"Father," said one of the rising generation to his paternal progenitor, "if I should call this cow's tail a leg, how many legs would she have?" "Why five, to be sure." "Why, no, father; would calling it a leg make it one?"'
Misattributed

Source: Lord of Chaos

Source: A Fire Upon the Deep (1992), Chapter 18 (p. 226).

Source: The Goose Girl

The New High Intensity Training (2004)