Question about the Ancient Ones.

[The Ancient One]

The Ancient Ones were mentioned in Cthulhu Dark Ages' mythos section as mainly what they and other gods could be interpreted as by other religions and cultures of that the time period, but they did not receive an entry in the current Malleus Monstrorum. Granted a number of creatures and deities didn't make the final cut in the MM, but it does seem odd to mention them and than not give them an entry. They were mentioned in Tawil at'Umr's (an avatar of Yog-Sothoth) entry, as speculation that might be an evolution of the Outer Gods and Old Ones. So if you were to include them in a campaign during the Dark Ages or any time period would you just modify their stats using the conversion guidelines or would it be up to the keeper to determine their stats?

I ask since in the spell section for Tawil at'Umr and the Ancient Ones in the first MM says they know all spells, while the current MM lists a few spells for Tawil at'Umr it also includes "others dealing with time and space at the Keeper's discretion".