Toby was clearly the name he used to gain the trust of the children he needed, but what was his true name? Judging by the messages written in Latin language like 'MEUS' it's possible it would be of a Latin origin, but what?
I don't think they ever came up with a definitive name. I think the guy who did the ritual with young Katie and Kristy said that Toby was just one of the names he had used over the years. I think it's much better when we don't know the name. Linda Blair's possession in The Exorcist would have lost a bit of its impact if they'd constantly mentioned that demon's name....Pazuzu. With a name like that, There's no way in hell that I could take it seriously.