Rashi: He knows what his animal and household need.

R. Yonah: After teaching that it is improper Silsul to refrain from working if he needs to, he teaches that a Tzadik will not refrain from any Mitzvah with his body due to Silsul, nor from any proper overseeing. He will not leave it for another to do. It is light in his eyes to oversee the poor, investigate their needs and set a table for them. Even for his animal, he supervises to feed it in its time. One may not taste anything ¹ before feeding his animals [that depend on him for food] -- "v'Nasati Esev b'Sadcha li'Vhemtecha v'Achalta v'Savata" (Devarim 11:15; Gitin 62a). Cruelty even to animals is a bad Midah - "v'Rachamav Al Kol Ma'asav" (Tehilim 145:9).

Moshav Zekenim (Shemos 9:27): The Tzadik is Hashem. He had had mercy on the flocks, and told the Egyptians to take them into the house to save them from the hail.

Orchos Tzadikim (Sha'ar ha'Achzariyus DH Gam): He does not overburden his animal, nor starve it.

Mesilas Yesharim (Perek 19): Part of Chasidus is not to pain any creation, even animals, and to have mercy on them.

Malbim: A Tzadik is one who acts according to the laws of Tzedek - not only with people, rather, even with his animal. He knows its nature, to give to it the proper amount of food at the proper time. He does not make it work more than its ability. Also this is a law of Chachmah and Tzedek! "V'Nasati Esev b'Sadcha li'Vhemtecha", and afterwards "v'Achalta v'Savata." Paining animals is a Torah Isur. We learn from Hashem's laws (how He made the world) - Hashem knows that a dog has little to eat, therefore He made that its food stays in its innards for three days (Shabbos 155b).

Malbim (according to Melitzah): Man is composed of two parts - the body is his animal, and the Nefesh is the man. A Tzadik gives to each his proper portion. When his desire entices him for the need of his animal, he knows Nefesh Behemto, i.e. the Neshamah, the Nefesh of the body (animal). The Nefesh of this animal, its desire is the opposite of the animal's desire. He kills the animal's desire in order to sustain the animal's Nefesh.

R. Yonah #1: Achzari is a noun (cruelty), like Achzariyus. The same applies to "Achzari v'Evrah" (Yeshayah 13:9).

R. Yonah #2: Achzari is an adjective. It is [singular, even though Resha'im is plural, like] "Banos Tza'adah" (Bereishis 49:22).

R. Yonah #1: Also when a Rasha is merciful, he is cruel. If he gives Tzedakah to an Oni, it is in the way of cruelty. This can be in many ways. E.g. if the Oni eats in his house, he does not heed him or draw him close. He does not smile at him, amidst his [airs of] authority. He seats him in the lowest place, and treats him disgracefully. Similar to this, it was taught 'one can feed to his father choice fowl, and be punished for this (Kidushin 31, e.g. if he shows anguish over the expense). If he gives to him from his money or grain, he gives to him openly, like one who rules over him, without concern if he blushes; one who makes another blush has no share in the world to come (Avos 3:11). One who appeases [an Oni] is greater than all of them ¹ .

R. Yonah #2: He is not merciful. He gives only to boast, and not amidst compassion for the poor. The gift of Resha'im is called Rachamei Resha'im, because the gift is the act of the Midah or mercy, or because people who see it think that he gives amidst mercy. This is like "Chesed l'Umim Chatas" (14:34) - Tzedakah that the nations do is a sin for them, since it is only to be haughty ² .

Moshav Zekenim (Shemos 9:27): This refers to Pharaoh and his entourage. (Hashem told them to take the flock into the houses, to save them from the hail. They did not have mercy on them. ³ )

Orchos Tzadikim (Sha'ar ha'Rachamim DH Aval and Sha'ar ha'Achzariyus DH ha'Achzariyus): Mercy can be cruelty. If one gives Tzedakah to an Oni, and oppresses him and says 'I gave to you such and such - you must toil for me and serve me like according to all that I gave to you.' This Midah is not found in Tzadikim, only in Resha'im.

Malbim: A Rasha acts oppositely to the laws of Tzedek. Even if you see the Midah of mercy in him, it is from hidden cruelty in him. E.g. he has mercy on someone in his hand and does not kill him, in order to make him work for him. He has mercy on his animal and gives to it extra food, in order to make it work harder. The root of his mercy is his own benefit, because he needs the matter.

Malbim (according to Melitzah): Resha'im have mercy on their animals (bodies), and give to them all their desires, and cut off its Nefesh. This is great cruelty against himself and his spiritual Neshamah!