Polymorph should use the 3.7.0 system of tracking monsters you’ve seen, and only allow you to turn into monsters the hero has encountered rather than any monster the player knows about out-of-game. Possibly only for controlled polymorphs; uncontrolled polymorphs could still be able to turn you into a completely random monster you haven’t seen before (and presumably, turning into a random monster would inform you that it exists and allow you to turn into it on subsequent controlled polymorphs even if you never encountered one randomly).

For variants implementing a druid role with wildshape, this would follow the same rule.