The scent of his mother
I always found this story very touching. It is related in Hisham Ibn al-Kalbi’s book of horses, one of the earliest such compilations by Muslim authors:
We were by the coastline; a stallion was brought to be bred to his mother; he refused. So they brought her into a house, threw a cover the door, and put a blanket on her. When he was done covering her, he smelled the scent of his mother. He put his teeth on the root (asl) of his male organ, cut it off and died.
The story was related by al-Awza’i to Abu Yusuf to Ibn al-Kalbi.
There’s a similar story in Varro, except in that instance, the stallion turned on the groom afterwards and savaged him.