“McCthulhu may be a spawn of Xagthrath the Grumbling Rug.” I don’t want to create disharmony or anything, but does McCthulhu *look* anything like Xagthrath? After all, breeding always tells. Then again, discretion is the better part of valor, so if this shouldn’t be brought up, just forget it.
Hard to tell.
The grumbling rugs all have a variable appearance.
What would be an indicator is that Xagthrath scuttles along on the floor, lays still to appear to be a rug, hangs out behind appliances, and is an elder creature of legend.