Welcome Van-Helsing! looking forward to seeing what you make of it all.
Onto the topic of Mythos monsters, with the expansions we have around 50 different Lurkers, using the rules to equip them and skill them up should allow you to use most any model in your collection. That said, we have many more profiles written, these will appear over time in Fhtagn! (the newsletter of The Cult Of The Black Goat...join us!) Be released occasionally on the resource section of our website, get casually posted on this forum or in various print sources and finally be re-compiled when second edition is available.
That is a really long winded way of saying, "I don't know, exactly...". In specific, some notable creatures (Mi-Go for example) were left out of the first edition so that they could be thoroughly playtested. Many require additional rules that would need to be 'retrofitted' into the main rules (such as the alien technology usable by the Mi-Go), which again, will be done when the second edition is completed.
Always excited to see what others come up with as well...