Is Polymorph responsible for the lack of high CR beasts?

[Yunru]

You could also add a deadly sonic attack like a Sperm Whale.

What would you choose for the damage type? Thunder?

 

[Leatherhead]

What would you choose for the damage type? Thunder?

Yes. Thunder damage is just concussive sound.

 

[Warmaster Horus]

A significant percentage of high CR Beasts are druids, anyway.

 

[Satyrn]

I don't think I ever really looked at Polymorph before this thread, even though I've contemplated having my druid prepare it.

It's weird that I could Polymorph a goblin into a wolf, but I couldn't do the same to a kobold.