Pandas are neutral animal mobs, but you can't use leads on them, and so for consistency it would make sense that you shouldn't be able to use leads on Dolphins and Polar bears either.

The report:
You are able to use leads on Dolphins and Polar Bears, despite not being able to use them on Pandas. All of them are neutral animal mobs.

Other mobs that might be inconsistent:

1. Spiders (can't use leads on them even during the day)
2. Bee (can use)
3. Untamed Wolf (can use)
4. Untamed Llama (can use)
5. Trader Llama (can use)