Shipwrecked on Skull Island
There are two playable monster classes in Shipwrecked on Skull Island: Mindflayers and Snakemen both are of better than average power, limited to six levels, and have a much slower advancement rate than the human classes.
However you can’t start as either unless one of your human characters has explored a very specific and different place on the island. So if you manage to enter the Black Pyramid of Yig, you can retire your character and bring in a Snakeman.
By the same token, if you manage to survive an encounter with a Mindflayer, without becoming one of its thralls, you can retire your survivor and roll up a Mindflayer