That being said...it is a bit strange that in these stories these "Cosmic Gods" are so incomprehensible. Look up any book around cosmic horror a central tenant is characters losing their mind as they interact with these unknowable titans.
But why? We can wrap our heads around paradoxes, quantum mechanics, theoretical physics and other insanely complicated things. But, the moment we stare or sometimes even think about these creatures we got insane or die.
Often times in cosmic horror stories this is a common thought process that the characters have. Just what is so mind shattering about a big ugly squid?
There's nothing mind shattering about the incomprehensible. The madness comes from understanding. These things are older than time. Most people think of this as negative time and shrug. We measure time from the perspective of human history so negative time (ie BC) is used commonly. Someone more knowledgeable will tell you there is no such thing as before the beginning of time as there is no time to measure.
Then you see it. It is outside time. Not timeless. It is not a black hole that will exist beyond your understanding of infinity. It is not a part of physics and yet somehow manifest in our universe. You can say it looks like a squid-faced dragon or a seven winged lions with bleeding eyes rather than fur. It is neither of those things. That is just the closest approximation to the wrongness of it your vocabulary allows.
Douglas Adams once wrote that if a man had the ability to see how utterly pointless and insignificant his life was in the entirety of the universe it would drive him irreparably mad. Looking at an old one will let you see how utterly pointless and insignificant our whole universe is.
It's like putting a huge number into a weak calculator, or diving by zero. because we're smarter than the calculator, we can go around them. The old ones are the numbers, we are the calculators.
It isn't the creatures themselves that drives you insane, it's the sudden realization that everything you know about biology, life, humans, and our place in the universe is as wrong as an ant's understanding of its place.
(Photos courtesy of Creative Commons)