In Celtic religion the stag was a symbol for the god Cernunnos, "The Horned One". Cernunnos was often portrayed with antlers himself, and was a god of the forest and wild animals. He was also seen as a god of 'Plenty', and the large Celtic 'Cauldrons of Plenty' often featured deer motifs amongst their ornate decoration. The magnificent Gundestrup Cauldron, for example, shows an antlered man alongside a deer and other wildlife, though this is often regarded as a representation of Cernunnos, his pose in a half lotus position.
It was Celtic ritual to create altars as they were considered places where the divine and human worlds interacted. They were places of exchange, communication, and influence. The horns of an altar functioned as boundary markers, setting apart the sacred space that was the actual place of intersection of the divine and human spheres.