Monoceros & Summoner

I always thought Monoceros were beast in the bestiary. Although, there was a mob called Dark Xion. I remember a thread from a long time ago, were a player asked for a Monoceros jug pet for BST, and the dev replied "We cannot do that, because they are magical/elemental in nature" or something to that extend.

My question is, how come they have never been released as a Summon for SMN? They were a Summon in the series; a legit summon, not like a Tonberry, Bomb or Chocobo. It is because they overlap with Carbuncle, who is light?