It is unlikely that Lupus is (or was) a demon. More than likely Lupus is lesser deity. Lupus has a constellation that was revered and for a long time was connected with Centaurs. Because of this, Lupus is more than likely an honorary being, same going for Karkinos, the Cancer or crab, that attacked Herakles during one of his labors. Hera rose the crab to the heavens for that deed, being one of Herakles main enemies, for she abhorred Herakles as he was a bastard (literal, as in a child that was not her's in her union with Zeus).
I am not aware of the conditions of Lupus or Loupus, I would have to do more research on the subject. Most Pagan beings are spirits that take on aspects of the world that we live in and the desires and mindsets of others. Lupus may be as that. He may have been a wolf raised, but he may also be a patron of wolven people, those wanting to be werewolves or people deeply in love or connected to our lupine cousins.
What you need to understand is that the divine beings have energy that is spread and is interconnected with other names and purposes. Research hunter gods or goddesses or anything that may have to do with him and he may be recognized there. The Romans knew this and this is why many Greek deities were converted to their own mythos when they took over, because many Roman gods seemed juxtaposed, but almost identical to the Greeks.
More than anything, trust your instincts, your heart and what the energy says about this being. If you are truly aware, and I mean truly, of the gods that speak to you, and they recognize Lupus as a god, than they are more than likely correct in one aspect or another.