I'm sure both these hadith are authentic :
"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"
But this hadith says :
"The Prophet (peace_be_upon_him) 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 book 38 , Number 4449