Burden not an animal with more than it can bear. We, truly, have prohibited such treatment through a most binding interdiction in the Book. Be ye the embodiments of justice and fairness amidst all creation.
(Bahá’u’lláh, Kitáb-i-Aqdas, #187)

Educate the children in their infancy in such a way that they may become exceedingly kind and merciful to the animals. If an animal is sick they should endeavor to cure it; if it is hungry, they should feed it; if it is thirsty, they should satisfy its thirst; if it is tired, they should give it rest.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 374)

If we say that these are effects of powers which animals also have, and of the powers of the bodily senses, we see clearly and evidently that the animals are, in regard to these powers, superior to man. For example, the sight of animals is much more keen than the sight of man; so also is their power of smell and taste. Briefly, in the powers which animals and men have in common, the animal is often the more powerful. For example, let us take the power of memory: if you carry a pigeon from here to a distant country, and there set it free, it will return, for it remembers the way. Take a dog from here to the center of Asia, set him free, and he will come back here and never once lose the road. So it is with the other powers such as hearing, sight, smell, taste, and touch. (‘Abdu’l-Bahá, Bahá’í World Faith, p. 304-305)

It is not only their fellow human beings that the beloved of God must treat with mercy and compassion, rather must they show forth the utmost loving-kindness to every living creature. For in all physical respects, and where the animal spirit is concerned, the self-same feelings are shared by animal and man … The feelings are one and the same, whether ye inflict pain on man or on beast. There is no difference here whatever. And indeed ye do worse to harm an animal, for man hath a language, he can lodge a complaint, he can cry out and moan; if injured he can have recourse to the authorities and these will protect him from his aggressor. But the hapless beast is mute, able neither to express its hurt nor take its case to the authorities … Therefore it is essential that ye show forth the utmost consideration to the animal, and that ye be even kinder to him than to your fellow man. Train your children from their earliest days to be infinitely tender and loving to animals. If an animal be sick, let them try to heal it, if it be hungry, let them feed it, if thirsty, let them quench its thirst, if weary, let them see that it rests.
(‘Abdu’l-Bahá, Selections from the Writings of ‘Abdu’l-Bahá, p. 159)

Just as the animal is more noble than the vegetable and mineral so man is superior to the animal. The animal is bereft of ideality; that is to say, it is a captive of the world of nature and not in touch with that which lies within and beyond nature; it is without spiritual susceptibilities, deprived of the attractions of consciousness, unconscious of the world of God and incapable of deviating from the law of nature. It is different with man. Man is possessed of the emanations of consciousness; he has perception, ideality and capable of discovering the mysteries of the universe.
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 240)

Man is generally sinful and the animal is innocent; unquestionably one must be more kind and merciful to the innocent. The harmful animals, such as the bloodthirsty wolf, the poisonous snake and other injurious animals are excepted, because mercy towards these is cruelty to man, and other animals. For instance, if you show kindness to a wolf this becomes a tyranny to the sheep, for it may destroy an entire flock of sheep. If you give the opportunity to a mad dog it may be the cause of the destruction of a thousand animals and men. Therefore, sympathy to the ferocious animal is cruelty to the peaceful animal, so they should be done away with. To the blessed animals, however, the utmost kindness should be exercised: the more the better it will be.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 374)

The animal spirit cannot penetrate and discover the mysteries of things. It is a captive of the senses.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 260)

The physical sensibilities and instincts are common to animal and man. Man is, however, negligent of this reality and imagines that sensibility is peculiar to mankind, therefore he practices cruelty to the animal. In reality what difference is there in physical sensations! Sensibility is the same whether you harm man or animal: there is no difference. Nay, rather, cruelty to the animal is more painful because man has a tongue and he sighs, complains and groans when he receives an injury and complains to the government and the government protects him from cruelty; but the poor animal cannot speak, it can neither show its suffering nor is it able to appeal to the government. If it is harmed a thousand times by man it is not able to defend itself in words nor can it seek justice or retaliate.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 373-374)

Therefore one must be very considerate towards animals and show greater kindness to them than to man.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 374)

This sympathy and kindness is one of the fundamental principles of the divine kingdom. Ye should pay great attention to this question.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 373)

To blessed animals the utmost kindness must be shown, the more the better. Tenderness and loving-kindness are basic principles of God’s heavenly Kingdom. Ye should most carefully bear this matter in mind.
(‘Abdu’l-Bahá, Selections from the Writings of ‘Abdu’l-Bahá, p. 160).

What are the animals’ propensities? To eat, drink, wander about and sleep. The thoughts, the minds of the animals are confined to these. They are captives in the bonds of these desires. Man becomes a prisoner and slave to them when his ultimate desire is no higher than his welfare in this world of the senses. Consider how difficult for man is the attainment of pleasures and happiness in this mortal world. How easy it is for the animal. Look upon the fields and flowers, prairies, streams, forests and mountains. The grazing animals, the birds of the air, the fishes neither toil nor undergo hardships; they sow not, nor are they concerned about the reaping; they have no anxiety about business or politics—no trouble or worry whatsoever. All the fields and grasses, all the meadows of fruits and grains, all the mountain slopes and streams of salubrious water belong to them. They do not labor for their livelihood and happiness because everything is provided and made possible for them. If the life of man be confined to this physical, material outlook, the animal’s life is a hundred times better, easier and more productive of comfort and contentment. The animal is nobler, more serene and confident because each hour is free from anxiety and worriment; but man, restless and dissatisfied, runs from morn till eve, sailing the seas, diving beneath them in submarines, flying aloft in airplanes, delving into the lowest strata of the earth to obtain his livelihood—all with the greatest difficulty, anxiety and unrest. Therefore, in this respect the animal is nobler, more serene, poised and confident. Consider the birds in the forest and jungle: how they build their nests high in the swaying treetops, build them with the utmost skill and beauty—swinging, rocking in the morning breezes, drinking the pure, sweet water, enjoying the most enchanting views as they fly here and there high overhead, singing joyously—all without labor, free from worry, care and forebodings. If man’s life be confined to the elemental, physical world of enjoyment, one lark is nobler, more admirable than all humanity because its livelihood is prepared, its condition complete, its accomplishment perfect and natural.
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 184-185)

Whensoever thou dost examine, through a microscope, the water man drinketh, the air he doth breathe, thou wilt see that with every breath of air, man taketh in an abundance of animal life, and with every draught of water, he also swalloweth down a great variety of animals. How could it ever be possible to put a stop to this process? For all creatures are eaters and eaten, and the very fabric of life is reared upon this fact. Were it not so, the ties that interlace all created things within the universe would be unravelled.
(‘Abdu’l-Bahá, Selections from the Writings of ‘Abdu’l-Bahá, p. 157)

Ye must not only have kind and merciful feelings for mankind, but ye should also exercise the utmost kindness towards every living creature.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 373-374)