Cernunnos is a figure that’s shrouded in mystery. He seems to be the only divinity that wasn’t Romanised, maybe because there was no way he could be Romanised or found an equivalent to in Graeco-Roman belief systems. If we strip away all the conjecture, hypotheses and speculation, what we’re left with is not very much in terms of concrete or material evidence; nevertheless, I think it would be safe to say that whatever belief in Cernunnos entailed and whatever his forms may have been or meant to the Celtic-speaking peoples of the past, “a” Cernunnos is most definitely rooted in “something” Gaulish/Celtic – the very name, despite some debate, being seemingly good Gaulish: the -on- part of the name is comparable to Maponos, Epona and so on.
As an aside, the etymology of Cernunnos as the “Horned One” is not universally accepted, there is debate about whether or not we can justify this epithet on linguistic grounds and we only have very sparse archaeological evidence to back us up. On the other hand, I have actually been fortunate enough to visit the Pillar of the Boatmen in Paris, and there is without a shadow of a doubt a horned figure, with what appears to rings or torques on his antlers, and the inscription “_ernunnos” (the initial C has been lost, but was recorded in historical times).
Now for a bit of my own conjecture, which, I stress, is purely my own conjecture. We know that our spiritual Celtic ancestors were fond of wordplay, that’s why we’re here in a sense, and the reconstructed proto-Celtic word *karno might also refer to a tomb or burial mound as much as to the idea of a “horn”, furthermore, *karwo could also be a deer. The idea of the forest leading to the “Otherworld”, the idea of the burial mound being a gate to the “Otherworld” and the idea of returning to the sacred grove or nature, all resound with me and all shout “druidic”, however, I stress again that this is just my own perception.
I wonder, I just wonder if there was some Bardic wordplay and punning going on a very, very long time ago.
A good source on etymology: Etymological Dictionary of Proto-Celtic, Ranko Matasovic, Leiden (2009), can be found at archive.org