What do the Mi-Go think about the Great Race of Yith, who are very similar to themselves except far more powerful and somewhat less malevolent towards humans? Well, less malevolent in that they are perhaps less interested in us and so would have less cause to engage in horrific experimentation.
What do the Mi-Go in Delta Green think about the Shan, who are portrayed in that game as maybe the second-most interventionist extraterrestrial species on Earth?
What do the ghouls know of/think about any other sentient/intelligent Mythos races? Maybe they just don't care and others don't care about them, seeing as how they're kind of doing their own thing. Same thing with the Deep Ones, really.
And what of cats?