Mostly-harmless beasts with high HP?

[Chronos]

Polymorph is a popular spell, because it's versatile: You can either turn one of your allies into a powerful or otherwise useful form, or you can turn an enemy into something harmless.

Now, for use on an ally, it's usually pretty clear what you want: Something with a lot of damage, or a lot of HP, or some specific useful ability. Often, this will be something up at or near the CR limit of the spell.

For use on an enemy, meanwhile, the obvious choice would be something small and harmless: A slug or caterpillar or something, or if your table insists on something with explicit stats, a toad or rat or the like. But this has a downside: Creatures like that usually only have 1 HP, and the spell ends if the target is reduced to 0. This makes it harder to use AoE damaging spells, and intelligent enemies might even deliberately squish their ally to restore them to their normal form.

So for cases like that, it'd be useful to turn enemies into something that has a lot of HP, but not much else. Maybe some large herbivore? But those usually still have decent gore and/or stomp attacks.

What are some good forms for this?

[EggKookoo]

The Terrasque

[JackPhoenix]

Killer Whale. CR 3, so it can be used on pretty much on any significant enemy, 90 HP. It's not harmless, but it's got only 5' reach, and no land movement speed, so it'll stay where you drop it. It's also huge, which may be advantage or detriment, depending on the space you have available and your needs. And unlike sharks and similar, it won't suffocate.

Sperm Whale from IWD is similar, but gargantuan and with double HP. CR 8, though, so it's better suited for serious opposition.

[Bobthewizard]

Killer whale with a teammate casting sickening radiance is the best. Once they fail 6 out of 100 CON saves they die.

You might have to cast couple cure wounds to make sure the 24d10 damage doesn't knock them to 0 HP before they die. Likely not an issue with the Sperm whale.

[Chronos]

I don't think anyone in my group has Sickening Radiance, but that's a great idea.

If the enemy is already in melee range of an ally (like, say, the cleric who already has Spirit Guardians running), though, an orca might be suboptimal: They'd still need to take at least one attack of opportunity. Any ideas for then?

[stoutstien]

I had one player who used a sharpened hollow tube made from adamanite capped on one end to just trap the polymorphed foe and give it full cover.