Jose Gomes Pinto, 55, slashed the throat of Jaime Pires with a razor after discovering he had sexually violated his favourite donkey named Russo.
Pires was found dead in a pool of blood in Portugal's Proenca-a-Velha village, The Sun reported.
Known to have had sex regularly with farm animals, Pires started sleeping with them 12 years ago, media reports said.
Mayor Francisco Silva said the killing was "sad and regrettable".
Pinto has denied murder charges during an appearance in court.
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