Cernunnos

is a giant mythical creature that used serve the Holy Knight: Ardbeg before his death.

History
Cernunnos was at some point given to Ardbeg by Arthur Pendragon to serve him.

Dalflare Range arc
Cernunnos introduces himself along Ardbeg as "Kellie", his pet dog. His true form is revealed when the two confront Percival's group over the fate of the Dalflare Range's demons. Cernunnos attacks Percival, who is unable to harm his horns with either his power or the Dragon Handle. Then Sin instructs him to use both at the same time, managing to infuse his weapon with his magic giving him the power to cut both Cernunnos' horns. The beast tries to devour him while he's distracted, but Nasiens manages to put her to sleep with his Misty Rain: Belladonna.

After Ardbeg's death, Cernunnos appears disguised as Kellie next to Ardbeg's armor, supposedly saddened about its owner's death.

Abilities/Equipment
Cernunnos is capable of firing lightning bolts from its antlers with incredible destructive potential. These antlers are as tough as garnet and cannot be cut even with steel.

Trivia

 * Cernunnos is based on the mythological creature.