There is no prescribed punishment for one who has sexual intercourse with an animal.

Abu Dawud said: 'Ata is also so. Al Hakam said: I think he should be flogged, but the number should not reach the one of the prescribed punishment. Al-Hasan said: He is like a fornicator.

Abu Dawud said: THe tradition of 'Asim proves the tradition of 'Amr b. Abi 'Amr as weak.
Sunan Abi Dawud Book 40, Hadith 115
← Previous Hadith The Prophet (ﷺ) said: If anyone has sexual intercourse with an animal, kill him and kill it along with him. I (Ikrimah) said: I asked him (Ibn Abbas): What offence can be attributed to the animal/ He replied: I think he (the Prophet) disapproved of its flesh being eaten when such a thing had been done to it.

Abu Dawud said: This is not a strong tradition.

Sunan Abi Dawud Book 40, Hadith 114 Next Hadith → A man came to the Prophet (ﷺ) and made acknowledgment before him that he had committed fornication with a woman whom he named. The Messenger of Allah (ﷺ) sent someone to the woman and he asked her about it. She denied that she had committed fornication. So he gave him the prescribed punishment of lashes and left her.

Sunan Abi Dawud Book 40, Hadith 116