As for me, I see no reason not to have a 25% magic resistance inherent in all creatures native to or long accustomed to dwelling in The Land Beyond the Magic Mirror and in Dungeonland. To my way of thinking, the society here is no more difficult to accept than one in which dragons fly and breathe fire, lightning, or poisonous gas. In a society which magic and heroism of incredible magnitude are commonplace and the fantastic is ordinary, how can one begin to rate degrees of the fantastic? Is a mimic more fantastic than a 15' tall giant? Or are talking flowers more remarkable than a human who can cast a ball of fire?--Gary Gygax, The Land Beyond the Magic Mirror (1983)
Intriguingly, this comes from one of the last modules penned by Gary Gygax to be published by TSR. Given that it was written during a time period when TSR was wholeheartedly engrossed in the brandification of D&D, it's somehow reassuring to see that Gygax, who was as responsible for the brandification of the game as anyone, still retained the spirit of the little game he self-published nearly 10 years previously.