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Polymorph doesn’t say what?
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As opposed to mass polymorph or true polymorph which both explicitly say that you choose.
True but you should be able to choose a little. When things like woodland creature are so random( and I guess know polymorph) its hard to form tactics around become rather useless. If you have a mean DM it can be a hazard like turning into something useless and not contributing to combat or summoning something that ether dies immediately or just clogs the initiative not helping.
If you have a mean DM, get a new DM. D&D isn't an adversarial game where a DM plays to win or turn every player plan on their head like an evil genie. Even if they get to decide what kind of creature the players get, they should pick whatever would be the most fun to introduce to the scene, not whatever would be the worst for the players. If they can come up with something that isn't the most obvious good pick and surprises the players while being useful in their own way, it's a good pick. Not to mention, usually forcing the players to improvise parts of their strategy on the fly leads to more fun play than just letting them steamroll an encounter using a predetermined, infallible plan.