Everyone else already went into a deep dive into what they believe the spell means. So I'll keep mine short.

My answer is no, the player doesn't have to know the creature exists, nor does the player have to have seen the beast before.

In the first sentence of Wild Shape, it gives you the limitation that the player has to have seen the beast bshirt.

No where in the 5 paragraphs of Polymorph, does it states this, so you do not need to have seen it.

I'd even venture to say you could create your own beast. But then it wouldn't have a stat block, which ends up putting more stress on your dm to come up with one.

Everyone else already went into a deep dive into what they believe the spell means, so I'll keep mine short.

My answer is no, the player doesn't have to know the creature exists, nor does the player have to have seen the beast before.

In the first sentence of Wild Shape, it gives you the limitation that the player has to have seen the beast before.

Nowhere in the five paragraphs of Polymorph, does it states this, so you do not need to have seen it.

I'd even venture to say you could create your own beast, but then it wouldn't have a stat block which ends up putting more stress on your DM to come up with one.