The man, thought to be a local firefighter, slaughtered the Iberian wolf, one of only a few thousand in the wild, and posed with the carcass in a photograph which has since attracted viral outrage.
The Association for the Conservation and Study of the Iberian Wolf (ASCEL) republished the image which shows the man holding the dead wolf by its ears.
A spokesman for ASCEL said: “Some workers of the Aguilar de Campoo, firefighters, or at least one of them, seems to have dedicated himself to posing this way with the animal, which has spread in multiple forums.
“We don’t know the reason for this.
“We want to spread the incident so that it does not happen again.”
Commenting on the photo, Karim Perez said: “It is clear that this guy represents a whole group who feel joy with the death and suffering of animals.”
Another commenter, Maria Rives, said she hoped the man was investigated and fired.
She added: “There must be a certain degree of sadism, to take a picture, laughing, with a dead animal.”
According to a 2003 census, the Iberian wolf, a subspecies of grey wolf, has a total population of only 2,000 wolves.
This makes the animals, which live in the forests and plains of northern Portugal and north-western Spain officially endangered.