Mabon is the season in which the harvest is being gathered. It's also the time in which the hunt often begins -- deer and other animals are killed during the autumn in many parts of the world. In some Pagan and Wiccan traditions, the deer is highly symbolic, and takes on many aspects of the God during the harvest season.
For many Wiccans, the antlers of the stag are associated directly with the fertility of the God. The Horned God, in his many incarnations, often appears wearing a headdress of antlers. In some depictions, the horns grow directly from his head. Early Paleolithic cave art shows men wearing antlers on their heads, so it would appear that the horn or antler has long been a symbol of worship in some form or another. In Egyptian legend, many gods appear to wear a pair of horns on their head.
In some Pagan paths, there is a correlation between the shape of a pair of horns and the crescent moon. The image of a stag with a full moon between his antlers represents both the male (the antlers) and the female (the moon) aspects of the Divine.
Mabon is the time, in many areas, when hunting season begins. While many Pagans are opposed to hunting, others feel that they can hunt for food as our ancestors did. For many Pagans, equally as important as the idea of caring about animals is the concept of responsible wildlife management. The fact is, in some areas, wild animals such as whitetail deer, antelope, and others have reached the status of nuisance animal. If you're wondering about why Pagans hunt, be sure to read Pagans and Hunting.
In some Wiccan traditions, a popular Mabon chant to sing is entitled simply, Hoof and Horn, originally written by Ian Corrigan of Ár nDraíocht Féin. You can listen to an audio clip here: Hoof and Horn.