Midrash su Salmi 32:9

Rabbi Tanchuma opened [his discourse] in the name of R. Judan [who] said in the name of R. Ayyevu (Ps. 32:9), “’Do not be like a horse or a mule, without understanding….’²⁵PRK 3:3; cf. PR 12:3. Six things are stated about the horse:²⁶Pes. 113b. (1) It eats much and (2) excretes little. (3) It loves promiscuity, (4) loves battle, (5) has a haughty spirit, (6) and abhors sleep. Moreover, there are also some who say [that] the horse wants to kill its master in battle. (Ibid.:) ‘Do not be like a horse or a mule.’ The Holy One, blessed be He, said to Israel, ‘”Do not be like a horse of a mule,’ that has no understanding. In the case of a horse, when one adorns it or feeds it, it obstructs and bends its neck and kicks him; and so too, the mule. But you are not to be like that. Rather be careful to repay the good person for his goodness and to repay the evil person for his evil.’ Good to the good [is] (Deut. 23:8), ‘You shall not despise the Edomite, as he is your brother.’ Bad to the bad [is] (Deut. 25:17), ‘Remember what Amalek did to you.’”

Rabbi Judan opened <his discourse> in the name of R. Ayyevu (with Ps. 32:9): DO NOT BE LIKE A HORSE OR A MULE, WITHOUT UNDERSTANDING….³²Tanh., Deut. 6:6; PRK 3:3; cf. PR 12:3. Six things are stated about the horse:³³Pes. 113b. (1) It eats much and (2) excretes little. (3) It loves promiscuity, (4) loves battle, (5) has a haughty spirit, (6) and abhors sleep. Moreover, there are also some who say: The horse wants to kill its master in battle. (Ibid.:) DO NOT BE LIKE A HORSE OR A MULE <WITH A BRIDLE AND A BIT>. ITS TRAPPINGS ARE FOR HOLDING IT IN CHECK. In the case of such a horse, when one bridles it, it balks. When one feeds it barley, it balks. When one pacifies it, it balks. When one approaches it, it balks {i.e., kicks}. You are not to be like that. Rather be careful [to repay] the good person for his goodness and to repay the evil person for his evil. (Deut. 25:17:) REMEMBER WHAT AMALEK DID TO YOU.