Oh ok just now realised what you are actually asking. What Atri Smriti is saying is , that (santapana) is the punishment for having unnatural sexual intercourse with any female animals except a cow.
In case of a similar offence against a cow, the punishment is even more severe. And this is mentioned in just the previous verse of the same chapter. I am giving both the verses here.
Prajapatya is- laid down for holding sexual intercourse with a beast,
or with a harlot. By holding intercourse with a cow, one should
perform- a Chandrayana spoken of by Manu. (267)
By discharging semen into inhuman females except a cow, into a woman
in [her] menses, into others that have no female organs ( a man
or a eunuch), or into water one should perform the penance of
Atri smriti, Chapter 1, Verses 267, 268.
Santapana is relatively a lot easy to perform. Whereas, Chandrayana is a much more difficult prayaschitta or penance which involves prolonged fasting.
Definitions of Chandrayana and Samtapana penances from Manu Smriti are given below:
11.212. (Subsisting on) the urine of cows, cowdung, milk, sour milk, clarified butter, and a decoction of Kusagrass, and fasting during one
(day and) night, (that is) called a Samtapana Krikkhra.
11.216. If one diminishes (one’s food daily by) one mouthful during the dark (half of the month) and increases (it in the same manner)
during the bright half, and bathes (daily) at the time of three
libations (morning, noon, and evening), that is called a lunar penance